P-type GaN Films Research Articles

Critical Mg doping on the blue-light emission in p-type GaN thin films grown by metal–organic chemical-vapor deposition

The photoluminescence and photocurrent from p-type GaN films were investigated at temperatures of 30 and 297 K for various Mg-doping concentrations. At a low Mg-doping level, there exists a photoluminescence center of the donor and acceptor pair transition at the 3.28 eV band. This center correlates with the defects for a shallow donor of the VGa and for an acceptor of MgGa. The acceptor level shows a binding energy of 0.2–0.25 eV, which was observed by measuring the photocurrent signal at a photon energy of 3.02–3.31 eV. At a high Mg-doping level, we found a photoluminescence center of a deep donor and acceptor pair transition of the 2.76 eV blue band. This center is attributed to the defect structures of MgGa –VN for the deep donor and MgGa for the acceptor. For low-doped samples, thermal annealing provides an additional photocurrent signal for unoccupied deep acceptor levels of 0.87–1.35 eV above the valence band, indicating p-type activation.

Secondary ion mass spectrometry analysis of In-doped p-type GaN films

SIMS was used to investigate the isoelectronic In-doped p-type GaN films. The growth rate of the p-type GaN film decreased with increasing Mg and In doping. The Mg saturation in GaN was 3.55×10 19 atoms/cm 3. The role of In as surfactant was evaluated by varying In concentrations and it was observed that the surface appeared smooth with increasing In incorporation. The Mg solubility in p-type GaN improved to 0.0025% molar ratio of the GaN with In incorporation. The In concentration results observed in neutron activation analysis (NAA) were found to be higher by a factor of 2.88 than that observed in SIMS and can be attributed to the difference in sensitivity of the two techniques. Good linearity in the results was observed from both techniques.

Thermal quenching effect of an infrared deep level in Mg-doped p-type GaN films

The thermal quenching of an infrared deep level of 1.2–1.5 eV has been investigated on Mg-doped p-type GaN films, using one- and two-step annealing processes and photocurrent measurements. The deep level appeared in the one-step annealing process at a relatively high temperature of 900 °C, but disappeared in the two-step annealing process with a low-temperature step and a subsequent high-temperature step. The persistent photocurrent was residual in the sample including the deep level, while it was terminated in the sample without the deep level. This indicates that the deep level is a neutral hole center located above a quasi-Fermi level, estimated with an energy of EpF=0.1–0.15 eV above the valence band at a hole carrier concentration of 2.0–2.5×1017/cm3.

Ohmic and Rectifying Contacts to n and p-type GaN Films

ABSTRACTElectrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 × 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized.

Temperature-dependent hole transport in GaN

The transport properties of Mg-doped, p-type GaN films grown by MOCVD havebeen measured using Hall effect and resistivity measurements over atemperature range of 400-120 K. The mobility is found to increase slowlyover the temperature range of 400-150 K. Below this temperature themobility is seen to decrease rapidly, while the corresponding Hall carrierdensity goes through a minimum before increasing to lower temperatures. Theseresults have been analysed, using a two-band model. This incorporates a simplevalence band model, calculated using a relaxation time approximation, andadditional transport within an acceptor impurity band. A good fit has beenobtained self-consistently to both the mobility and carrier density over atemperature range of 400-120 K. We find that neutral scattering plays animportant role in limiting the hole mobility.

Effect of dry etching conditions on surface morphology and optical properties of GaN films in chlorine-based inductively coupled plasmas

Dry etching of undoped, n- and p-type GaN films has been carried out in a planar type inductively coupled plasma (ICP) system. The effect of etching conditions on surface morphology and optical properties of GaN films etched in Cl2/Ar discharges was studied. All the GaN films showed overall similar etching behavior at 700 W ICP, 150 W rf, 40 mTorr, and 25% Cl2. The surface roughness was relatively independent of the rf power up to 150 W, resulting in quite smooth morphology (root-mean-square roughness 1.1–1.3 nm), while etching at higher chuck powers (>200 W) produced rougher surfaces due to increased ion bombardment. The lattice disorder and point defect density were lower during the ICP etching compared with reactive ion etching. The intensity of the band edge peak of as-grown n-GaN was decreased after the ICP etching, and the extent of intensity decrease was inversely proportional to the applied chuck power. By contrast, the as-grown p-GaN showed a weak intensity of band edge emission, but the peak shifted to 460 nm after ICP etching with substantial increase in photoluminescence intensity.

Electron and hole dynamics in GaN

The hot electron and hot hole dynamics have been studied in n-type and p-type GaN films grown by molecular beam epitaxy on sapphire. A novel non-degenerate femtosecond pump-probe spectroscopic technique has been used, in which the electrons (or holes) are excited by a strong infrared pump, and carrier thermalization and cooling are monitored by a tunable UV probe. From the complex transients, which show bleaching and induced absorption, we have been able to determine several parameters of importance, including the hot electron and the hot hole relaxation times. The experimental results are compared to the theoretical predictions obtained assuming that LO-phonon emission is the dominant energy relaxation process.

Depth-Resolved Cathodoluminescence of III–V Nitride Films Grown by Plasma-Assisted Molecular Beam Epitaxy

The luminescence properties of III–V nitride films (an undoped GaN, an undoped GaN/Al0.2Ga0.8N multiquantum well, and a p-type Mg-doped GaN films) were investigated using the depth-resolved cathodoluminescence (CL) spectroscopy. From the low- and room-temperature CL of the undoped GaN and GaN/AlGaN MQW films, an emission at 2.9 eV was found with the longer penetration depth and is attributed to the higher density of dislocations at the interface between the film and the substrate. In order to explain the depth-resolved CL of the Mg-doped GaN film, the configuration coordinate model is proposed on the basis of the local strain near the Mg impurities. This model demonstrates that local strain may play a crucial role in controlling the radiative efficiency, line width, and peak position of the luminescence from the film.

Effects of a two-step rapid thermal annealing process on Mg-doped p-type GaN films grown by metalorganic chemical vapor deposition

A two-step rapid thermal annealing (RTA) process is proposed in order to improve the electrical properties, the crystal quality, and the surface roughness of Mg-doped p-type GaN films. In the two-step RTA process, the first low temperature step (600 °C) with a long annealing time (5 min) was followed by the second high temperature (950 °C) step with a short annealing time. These results show that the two-step RTA process significantly improves electrical properties and reduces the surface roughness of p-GaN compared to the one-step RTA process.

Cl 2 -based dry etching of GaN films under inductively coupled plasma conditions

Dry etching of undoped, n- and p-type GaN films was carried out in Cl2-based inductively coupled plasmas (ICPs) using different rf excitation frequencies of 100 kHz and 13.56 MHz, in which the rf chuck power source operates. The etch rates with lower frequency of 100 kHz are somewhat greater than those with a higher frequency of 13.56 MHz due to higher ion bombarding energy with lower frequency. The highest etch rates with the 100 kHz frequency were obtained at moderately high ICP power of 700 W: ∼9300 Å/min of n-GaN, ∼5300 Å/min of p-GaN, and ∼7100 Å/min of undoped GaN. The 13.56 MHz frequency of rf chuck power source produced maximum etch rates of ∼7900 Å/min of n-GaN, ∼5800 Å/min of p-GaN, and 6100 Å/min of undoped GaN at 20 mTorr, 700 W ICP, and 150 W rf power. The surface roughness was relatively independent of the chuck power up to 150 W in 13.56 MHz and showed fairly smooth morphology (rms 1.1–1.3 nm), while etching at higher rf power (>200 W) produced rougher surface.

Hot hole relaxation dynamics in p-GaN

The hot hole relaxation dynamics is studied in a Mg-doped p-type GaN film grown by molecular-beam epitaxy on sapphire. A nondegenerate femtosecond pump-probe technique is used, in which the holes are excited by an infrared pump and the hole dynamics is monitored by a tunable near ultraviolet probe. Complex transients, showing bleaching or induced absorption, are observed. A hot hole energy relaxation time of 0.6 ps has been obtained. Modeling suggests that longitudinal optical phonon emission modified by hot phonon effects is the dominant energy relaxation process.

The doping process of p-type GaN films

The formation of p-type GaN film is a key technology in developing optoelectronic devices. P-type doping (concentration∼10 17 cm −3) has been achieved in grown GaN by metalorganic chemical vapor deposition (MOCVD) with Mg (CP 2Mg) doping. The Hall measurement results indicate that Mg diffused GaN films also have p-type conductivity with carrier concentration about 10 17 cm −3 and a mobility of 10 cm 2 V −1 s −1. For the as-grown Mg-doped GaN, the room temperature photoluminescence (PL) spectra show a blue emission peak around 420–450 nm, and the spectral peak depends on the carrier concentrations. For Mg-diffused GaN, the PL spectra shows only a broad violet emission for samples diffused at 900–1100°C.

Reactivation of Mg acceptor in Mg-doped GaN by nitrogen plasma treatment

Mg-doped GaN films, grown by metalorganic chemical vapor deposition, were treated with a nitrogen plasma after a conventional rapid thermal annealing (RTA). The conductivity of the p-type GaN film was greatly enhanced by nitrogen plasma treatment, and exhibited a higher sheet hole concentration as well as lower sheet resistance than the RTA samples. A photoluminescence (PL) band which peaked at 3.27 eV was new, and a band at 2.95 eV was markedly attenuated in the plasma treated samples. PL measurements suggest that self-compensation in a Mg-doped GaN caused by the nitrogen vacancies is effectively reduced by the nitrogen plasma treatment, leading to an enhanced p-type conductivity. In addition, the plasma-treated sample revealed a drastic reduction in specific contact resistance by three orders of magnitude, compared with the RTA samples.

Contacts of titanium nitride to n- and p-type gallium nitride films

The electrical characteristics of titanium nitride (TiN x ) contacts to n- and p-type GaN films, deposited by reactive magnetron sputtering at room temperature, are investigated. The contacts of TiN x to n-type GaN are ohmic and to p-type GaN are rectifying, while their properties are strongly dependent on the stoichiometry of the deposited titanium nitride layer. It is shown that the ohmic behavior of the contacts is associated with the presence of a high density of interface states and not to the low Schottky barrier.

High-transparency Ni/Au ohmic contact to p-type GaN

In this study, a very thin Ni/Au bilayer metal film was prepared by electron beam evaporation and thermal alloying to form ohmic contact on p-type GaN film. After thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film exhibited ohmic behavior. The Ni/Au contacts showed a specific contact resistance of 1.7×10−2 Ω cm2 at an alloying temperature of 450 °C. In addition, the light transmittance of the Ni/Au (2 nm/6 nm) bilayer on p-type GaN was measured to be around 85% at 470 nm. These results suggest that a suitable metallization technology for the fabrication of light emitting devices can be achieved.

Photocurrent Spectroscopy Investigations of Mg-Related Defects Levels in p-Type GaN

AbstractThe defect levels associated with Mg impurity in p-type GaN films were systematically investigated in terms of doping concentration by photocurrent spectroscopy. Mg-doped GaN samples were grown on sapphire substrate by metal organic chemical vapor deposition and annealed in nitrogen atmosphere at 850 for 10 minutes. At room temperature, PC spectra showed two peaks at 3.31 and 3.15 eV associated with acceptor levels formed at 300 and 142 meV above valence band in as grown samples. But, after the thermal annealing, PC spectra exhibited various additional peaks depending on the Mg concentration. In the GaN samples with Mg concentration around 6 7 1017 cm−3, we have observed PC peaks related to Mg at 3.31 as well as 3.02 eV and carbon acceptor at 3.17 eV. For moderately Mg doped GaN samples, i.e., the hole concentration p=3 4 1017 cm−3, additional peak was observed at around 0.9 eV which can be attributed to defects related to Ga vacancy. For relatively low Mg doped samples whose hole concentrations are 1 2 1017 cm−3, additional broad peak was observed at around 1.3 eV. This peak may be related to the yellow band luminescence. As the Mg concentration is increased, the concentration of Ga vacancies can be reduced because Mg occupies the substitutional site of Ga in GaN lattice. When the hole concentration is above 6 7 1017 cm−3, the yellow luminescence and Ga vacancy related peaks disappeared completely.

Photoluminescence spectroscopy of Mg-doped GaN

We have grown Mg-doped GaN films by metalorganic chemical vapor deposition with various CP2Mg flow rates. After 750 °C postgrowth annealing, p-type GaN films with carrier concentrations and mobilities about 2×1017/cm3 and 10 cm2/V s, respectively, have been achieved. A dominant photoluminescence (PL) line around 2.9 eV was observed at room temperature. By studying the dependence of PL on excitation density at 20 K, the emission line around 2.95 eV can be attributed to a donor-to-acceptor pair transition rather than to a conduction band-to-impurity transition involving the Mg-related deep level. We suggest that the DAP transition line is caused by a Mg related deep level at about 510 meV above the valence band. It is much deeper than the acceptor level at 250 meV commonly produced by the Mg dopants.

Growth condition dependence of Mg-doped GaN film grown by horizontal atmospheric MOCVD system with three layered laminar flow gas injection

We developed a novel atmospheric pressure horizontal MOCVD system (SR2000) for the growth of III-nitride film. This system was designed for high-speed gas flow in order to suppress thermal convection and undesirable reactant gas reaction. We have grown Mg-doped GaN films using SR2000. We studied the bis-cyclopentadienyl magnesium (Cp2Mg) flow rate dependence and growth temperature (Tg) dependence of Mg-doped GaN. As a result, we have obtained p-type GaN film with hole carrier density of 8×1017cm−3 with a mobility of 7.5cm2/(Vs) at the growth condition with Cp2Mg flow rate of 0.1μmol/min at Tg of 1025°C.

P -type conduction in as-grown Mg-doped GaN grown by metalorganic chemical vapor deposition

We have clarified the effect of H2 and NH3 on the passivation of Mg acceptor in p-type GaN films grown by metalorganic chemical vapor deposition. It has been found that the small amount of H2 carrier gas strongly influences the electrical property of the Mg-doped GaN films. Low-resistivity p-type GaN has been obtained by H2-free growth without any post-treatments. Its acceptor concentration is as high as that obtained by conventional H2-rich growth with subsequent thermal annealing. It has also been clarified that hydrogen produced by NH3 dissociation does not prevent Mg from electrically activating in H2-free growth.

The effect of thermal annealing on the Ni/Au contact of p-type GaN

In this study, the Ni/Au layers prepared by electron beam evaporation and thermal alloying were used to form Ohmic contacts on p-type GaN films. Before thermal alloying, the current–voltage (I–V) characteristic of Ni/Au contact on p-type GaN film shows non-Ohmic behavior. As the alloying temperature increases to 700 °C, the I–V curve shows a characteristic of Ohmic contact. The Schottky barrier height reduction may be attributed to the presence of Ga–Ni and Ga–Au compounds, such as Ga4Ni3, Ga3Ni2, GaAu, and GaAu2, at the metal-semiconductor interface. The diffusing behavior of both Ni and Au have been studied by using Auger electron spectroscopy and Rutherford backscattering spectrometry. In addition, x-ray diffraction measurements indicate that the Ni3N and Ga4Ni3 compounds were formed at the metal-semiconductor interface.