P-type GaN Films Research Articles

Negligible carrier freeze-out facilitated by impurity band conduction in highly p-type GaN

Highly p-type GaN films with hole concentrations exceeding 6 × 1019 cm−3 grown by metal-modulated epitaxy are electrically characterized. Temperature-dependent Hall effect measurements at cryogenic temperatures reveal minimal carrier freeze-out in highly doped samples, while less heavily doped samples exhibited high resistivity and donor-compensated conductivity as is traditionally observed. Effective activation energies as low as 43 meV were extracted, and a maximum Mg activation efficiency of 52% was found. In addition, the effective activation energy was found to be negatively correlated to the hole concentration. These results indicate the onset of the Mott-Insulator transition leading to impurity band conduction.

Piezo-Phototronic Effect on Electroluminescence Properties of p-Type GaN Thin Films

We present that the electroluminescence (EL) properties of Mg-doped p-type GaN thin films can be tuned by the piezo-phototronic effect via adjusting the minority carrier injection efficiency at the metal-semiconductor (M-S) interface by strain induced polarization charges. The device is a metal-semiconductor-metal structure of indium tin oxide (ITO)-GaN-ITO. Under different straining conditions, the changing trend of the transport properties of GaN films can be divided into two types, corresponding to the different c-axis orientations of the films. An extreme value was observed for the integral EL intensity under certain applied strain due to the adjusted minority carrier injection efficiency by piezoelectric charges introduced at the M-S interface. The external quantum efficiency of the blue EL at 430 nm was changed by 5.84% under different straining conditions, which is 1 order of magnitude larger than the change of the green peak at 540 nm. The results indicate that the piezo-phototronic effect has a larger impact on the shallow acceptor states related EL process than on the one related to the deep acceptor states in p-type GaN films. This study has great significance on the practical applications of GaN in optoelectronic devices under a working environment where mechanical deformation is unavoidable such as for flexible/printable light emitting diodes.

Comparison of surface photovoltage behavior for n-type versus p-type GaN

Using a Kelvin probe, the authors have studied changes in surface contact potential during illumination, i.e., surface photovoltage (SPV), for n- and p-type GaN films grown by hydride vapor phase epitaxy. Short ultraviolet (UV) exposures (3 s) generate a positive SPV of about 0.5 eV for n-type and a negative SPV of about −0.6 eV for p-type GaN, which is consistent with the expected surface band bending for these two surface types. The fast component of the SPV is attributed to the accumulation of photogenerated holes (n-type) or electrons (p-type) at the surface, which results in a decrease in band bending. During long UV exposures (1 h), however, slower photoinduced processes can cause the SPV signal to change as a function of the ambient conditions. For both n- and p-type GaN, UV illumination causes the adsorption of negatively charged oxygen species on the surface in air and their subsequent desorption in vacuum. It appears that this adsorbate effect is most prevalent for n-type GaN in vacuum and p-type GaN in air, with significant changes in the SPV of up to 0.3 eV (n-type) and 0.6 eV (p-type). One important difference between the GaN films is the sensitivity of p-type GaN to the illumination geometry. When the sample contacts are exposed to near-bandgap light, unexpected offsets of the SPV signal can result. If such offsets are appropriately taken into account, however, then the SPV behaviors for both n- and p-type GaN can be reasonably modeled by thermionic expressions.

Effects of hydrogen treatment on ohmic contacts to p-type GaN films

This study investigated the effects of hydrogen (H 2) treatment on metal contacts to Mg-doped p-GaN films by Hall-effect measurement, current–voltage ( I– V) analyzer and X-ray photoemission spectra (XPS). The interfacial oxide layer on the p-GaN surface was found to be the main reason for causing the nonlinear I– V behavior of the untreated p-GaN films. The increased nitrogen vacancy (V N) density due to increased GaN decomposition rate at high-temperature hydrogen treatment is believed to form high density surface states on the surface of p-GaN films. Compared to untreated p-GaN films, the surface Fermi level determined by the Ga 2p core-level peak on 1000 °C H 2-treated p-GaN films lies about ∼2.1 eV closer to the conduction band edge (i.e., the surface inverted to n-type behavior). The reduction in barrier height due to the high surface state density pinned the surface Fermi level close to the conduction band edge, and allowed the electrons to easily flow over the barrier from the metal into the p-GaN films. Thus, a good ohmic contact was achieved on the p-GaN films by the surface inversion method.

Optimal activation condition of nonpolar a-plane p-type GaN layers grown on r-plane sapphire substrates by MOCVD

The optimal conditions of p-type activation for nonpolar a-plane (1 1 –2 0) p-type GaN films on r-plane (1 –1 0 2) sapphire substrates with various off-axis orientations have been investigated. Secondary ion mass spectrometry (SIMS) measurements show that Mg doping concentrations of 6.58×10 19 cm −3 were maintained in GaN during epitaxial growth. The samples were activated at various temperatures and periods of time in air, oxygen (O 2) and nitrogen (N 2) gas ambient by conventional furnace annealing (CFA) and rapid thermal annealing (RTA). The activation of nonpolar a-plane p-type GaN was successful in similar annealing times and temperatures when compared with polar c-plane p-type GaN. However, activation ambient of nonpolar a-plane p-type GaN was clearly different, where a-plane p-type GaN was effectively activated in air ambient. Photoluminescence shows that the optical properties of Mg-doped a-plane GaN samples are enhanced when activated in air ambient.

Self‐Powered, Ultrafast, Visible‐Blind UV Detection and Optical Logical Operation based on ZnO/GaN Nanoscale p‐n Junctions

Ultrafast-response (20 μs) UV detectors, which are visible-blind and self-powered, in devices where an n-type ZnO nanowire partially lies on a p-type GaN film, are demonstrated. Moreover, a CdSe-nanowire red-light detector powered by a nanoscale ZnO/GaN photovoltaic cell is also demonstrated, which extends the device function to a selective multiwavelength photodetector and shows the function of an optical logical AND gate.

Single ZnO Nanowire/p‐type GaN Heterojunctions for Photovoltaic Devices and UV Light‐Emitting Diodes

We fabricate heterojunctions consisting of a single n-type ZnO nanowire and a p-type GaN film. The photovoltaic effect of heterojunctions exhibits open-circuit voltages ranging from 2 to 2.7 V, and a maximum output power reaching 80 nW. Light-emitting diodes with UV electroluminescence based on the heterojunctions are demonstrated.

Room-temperature ferromagnetism and in-plane magnetic anisotropy characteristics of nonpolar GaN:Mn films

Diluted magnetic nonpolar GaN:Mn films have been fabricated by implanting Mn ions into nonpolar a-plane ( 1 1 2 ¯ 0 ) p-type GaN films and a subsequent rapid thermal annealing process. The ferromagnetism properties of the films were studied by means of superconducting quantum interference device (SQUID). Clearly in-plane magnetic anisotropy characteristics of the sample at 10 K were revealed with the direction of the applied magnetic field rotating along the in-plane [0 0 0 1]-axis. Moreover, obvious ferromagnetic properties of the sample up to 350 K were detected by means of the temperature-dependent SQUID.

Thermal conductivity of semi-insulating, p-type, and n-type GaN films on sapphire

The thermal conductivity of undoped, n-type, and p-type GaN films deposited on (0001) substrates of sapphire was measured by the 3-ω method in the temperature range between 215 and 300K. The thickness, thermal conductivity, and heat capacity of the individual layers were used to simulate the experimental value of the increment in temperature of the heater using a multilayer model. The thermal conductivity of undoped GaN film was found to be much higher than that of p-type film. Also, the thermal conductivity of n-type GaN film was slightly smaller than that of p-type film. Modeling of the temperature dependence of the thermal conductivity in the films showed that phonon-dopant and three-phonon umklapp scattering are important. Smaller thickness and hence smaller volume fraction of the film with lower dislocation density was also found to be responsible for lower thermal conductivity in n- and p-type GaN films.

Enhancement of blue emission from GaN films and diodes by water vapor remote plasma treatment

We have studied the effects of water vapor remote plasma treatment (H 2O RPT) on emission from Si-doped and undoped n-type GaN films grown on c-face sapphire substrates by the atmospheric-pressure metalorganic chemical vapor deposition method. We found by photoluminescence (PL) spectroscopy at 77 K that H 2O RPT enhanced violet emission from the n-type GaN films, similarly to the previously observed enhancing effect on blue emission from Mg-doped p-type GaN films. We also observed that H 2O RPT reduced the intensity of ESR signal comprising of two peaks detected at 4.2 K in undoped n-type films and increased their resistivity measured at room temperature. We ascribe the enhancing effect of H 2O RPT to the passivation of non-radiative recombination centers by hydrogen produced by H 2O RPT. We formed GaN p–n diodes from the layer structure of p-GaN:Mg/n-GaN:Si/sapphire, and carried out electroluminescence (EL) measurements of the diodes at 77 K. We observed that EL intensity was also enhanced by H 2O RPT.

Activation of Mg-doped P-GaN by using two-step annealing

One- and two-step rapid thermal annealing (RTA) for activating Mg-doped p-type GaN films had been performed to compare with conventional furnace annealing (CFA). The two-step annealing process consists of two annealing steps: the first step is performed at 750°C for 1min and the second step is performed at 600°C for 5min in pure O2 or air ambient. It is found that the samples annealed in air ambient exhibit poor electrical properties as compared to those annealed in pure O2. Compared to one-step RTA annealing and CFA annealing, the samples with two-step annealing exhibit higher hole concentration and lower resistivity. This means that the two-step annealing is a powerful method to enhance the electrical performance of Mg-doped p-type GaN films. Similar results were also evidenced by photoluminescence (PL) measurement. Possible mechanism was confirmed by secondary ion mass spectrometry analysis.

Epitaxial ScAlMgO4(0001) films grown on sapphire substrates by flux-mediated epitaxy

ScAlMgO4(0001) epitaxial films have been synthesized on sapphire (0001) substrates by flux-mediated epitaxy. The key points were high temperature deposition and the use of additive BiOx, which was simultaneously supplied during the deposition; otherwise the growth of spinel phase such as MgAl2O4 was dominant. The BiOx is thus considered to promote the growth of ScAlMgO4 like a flux. This result indicates that the flux-mediated epitaxy is a promising way to high quality single crystal ScAlMgO4 films as a lattice-matched substrate for p-type ZnO and GaN films.

EFFECT OF RAPID THERMAL ANNEALING ON THE Mg-DOPED GaN/Si FILM

Mg -doped GaN films have been successfully prepared on Si (111) substrate by metal-organic chemical vapor deposition (MOCVD). Upon rapid thermal annealing (RTA) treatment, the films showed p-type conductivity with a carrier density of 7.84 cm-3, a mobility of 5.54 cm2V-1s-1, and a resistivity of about 0.144 Ωcm, which were much better than that of the films without rapid thermal annealing (RTA) treatment. It was found that the surface morphology and crystal quality of the obtained p-type GaN films were greatly improved by RTA treatment, while the residual stress and dislocations in these films were decreased.

Measurement of temperature-dependent defect diffusion in proton-irradiated GaN(Mg, H)

Deuterated p-type GaN(Mg,H2) films were irradiated at room temperature with 1MeV protons to create native point defects with a concentration approximately equal to the Mg doping (5×1019cm−3). The samples were then annealed isothermally at a succession of temperatures while monitoring the infrared absorption due to the H local mode of the MgH defect. As the samples were annealed, the MgH absorption signal decreased and a new mode at slightly higher frequency appeared, which has been associated with the approach of a mobile nitrogen interstitial. We used the time dependence of the MgH absorption to obtain a diffusion barrier of the nitrogen interstitial in p-type GaN of 1.99eV. This is in good agreement with theoretical calculations of nitrogen interstitial motion in GaN.

Effects of Thermal Annealing on Al-Doped ZnO Films Deposited on p-Type Gallium Nitride

In this study, Al-doped ZnO (AZO) and films were deposited on p-type GaN films followed by thermal annealing to form ohmic contacts. After thermal annealing, the resistivities of AZO films reduced from to . Both as-deposited AZO and contacts on p-GaN displayed a non-ohmic characteristic. Only the 800°C-annealed contacts exhibited a linear current–voltage characteristic, showing a specific contact resistance of around . After undergoing the annealing in nitrogen ambience, the light transmittance of the films increased from 70% to higher than 90% in the visible range. These results revealed that the contact can serve as a suitable transparent current spreading layer for the fabrication of GaN-based light-emitting devices.

A normally off GaN n-MOSFET with Schottky-barrier source and drain on a Si-auto-doped p-GaN/Si

We have fabricated an enhancement-mode n-channel Schottky-barrier-MOSFET (SB-MOSFET) for the first time on a high mobility p-type GaN film grown on silicon substrate. The metal contacts were formed by depositing Al for source/drain contact and Au for gate contact, respectively. Fabricated SB-MOSFET exhibited a threshold voltage of 1.65 V, and a maximum transconductance(g/sub m/) of 1.6 mS/mm at V/sub DS/=5V, which belongs to one of the highest value in GaN MOSFET. The maximum drain current was higher than 3 mA/mm and the off-state drain current was as low as 3 nA/mm.

Enhanced light output of GaN-based power LEDs with transparent Al-doped ZnO current spreading layer

In this study, Al-doped ZnO (AZO) Ni-AZO and NiO/sub x/--AZO films were deposited on p-type GaN films. The depositions were followed by thermal annealing to form Ohmic contacts. The p-GaN-AZO contacts exhibited a non-Ohmic electrical characteristic. However, electrical characteristic could be greatly improved by insertion of Ni or NiO/sub x/ between AZO film and p-GaN layer. In case of 1/spl times/1 mm/sup 2/ ultraviolet light-emitting diodes (LEDs) with Ni-AZO contacts, the light output approached saturation point when the injection current was about 400 mA. However, the saturation point was as high as 500 mA for the LEDs with NiO/sub x/--AZO contacts. The lower saturation point could be due to the fact that the resistivity of Ni-AZO films was higher than that of NiO/sub x/--AZO films, thus leading to a severe current crowding effect. The increased resistivity of the Ni-AZO films could be attributed to the interdiffusion between Ni and AZO films. When compared to the LEDs with Ni-Au Ohmic contacts, the light output of the LEDs with Ni-AZO and NiO/sub x/--AZO contacts was higher by 38.2% and 60.6% at 350 mA, respectively.

Highly p-Type a-GaN Grown on r-Plane Sapphire Substrate

AbstractMg-doped p-type a-plane GaN films were grown on unintentionally doped a-plane GaN templates by metalorganic vapor phase epitaxy (MOVPE). The Mg concentration in a-plane GaN increased with increasing Mg source gas flow rate. A maximum hole concentration of 2.0 × 1018 cm-3 with a hole mobility of 4.5 cm2/Vs and resistivity of 0.7 Ω·cm were achieved. The activation ratio was 5.0 × 10-2. It was found that a maximum hole concentration in p-type a-plane GaN was higher than that in p-type c-plane GaN. The activation energy of Mg acceptors in p-type a-plane GaN with the maximum hole concentration was found to be 118 meV by temperature-dependent Hall-effect measurement.

Efficient p-type doping of GaN films by plasma-assisted molecular beam epitaxy

In this paper we report on the mechanism of efficient incorporation of Mg in GaN films during growth by plasma-assisted molecular beam epitaxy. It is found that Mg incorporates more efficiently during growth of GaN films at high temperatures (770 °C) under extreme Ga-rich conditions. We propose that this result is due to the dissolution of Mg in the excess Ga on the growth surface and its incorporation into the GaN film via liquid-phase processes. Transport measurements at 300K together with secondary-ion-mass-spectroscopy indicate that the Mg-doping efficiency of GaN under these conditions of growth is 10%. Using this method of doping, p-type GaN films free of Ga droplets, with hole concentrations varying from 2×1017 to 3×1018cm−3 and corresponding mobilities varying from 30 to 2cm2∕Vs, are obtained. The lowest resistivity achieved is 0.3 Ω cm.

Si diffusion in p-GaN

The characteristics of p-type Mg-doped GaN films diffused with Si are studied. N-type conductivity is achieved, and the carrier mobility of diffused GaN is 90–150 cm2 V−1 s−1, higher than that of p-GaN but less than that of epitaxially grown n-GaN. The Mg acceptor states could become deep compensating defects, and the compensation ratio NA/ND is 0.3, 0.45, 0.6, and 0.75 for 800, 900, 1000, and 1100 °C diffused GaN, respectively. The carrier transport may be dominated by electron hopping through these deep compensating centers or through diffusion. The results of temperature-dependent carrier concentration indicate that thermal annealing may induce defects at the surface, leading to an additional activation energy Ed∼10 meV in the 200–500 K region in diffused GaN.