p-GaN Films Research Articles

Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices

Electrical and optical properties of modulation-doped p-AlGaN/GaN superlattices are compared to those of similarly doped p-GaN films. It is shown that modulation doping increases the sheet hole concentration by several times. In p-AlGaN/GaN superlattices grown on GaN underlayers, this increase is accompanied by a significant increase in hole mobility which results in a remarkable decrease in sheet resistivity of the structure compared to p-GaN films and this decrease in sheet resistivity should hold up to temperatures exceeding 350 °C. For superlattices prepared on AlGaN underlayers, the mobility decreases compared to p-GaN. In such superlattices, one also observes a strong redshift and a strong broadening of the band edge luminescence peak coming most probably from increased mosaicity and strain which would also explain the observed deterioration of mobility. The magnitude of the redshift in the position of the band edge luminescence band slightly increased upon application of reverse bias which is interpreted as a manifestation of quantum-confined Stark effect.

Electronic states in modulation doped p-AlGaN/GaN superlattices

The properties of p-AlGaN/GaN modulation doped superlattices (SLs) prepared by molecular beam epitaxy were studied by means of conductivity versus temperature, admittance spectroscopy, photoinduced current spectroscopy, microcathodoluminescence (MCL) spectra measurements, and measurements of effective diffusion lengths. It is shown that in SLs grown on GaN underlayers the sheet resistivity is about two orders of magnitude lower than for reference p-GaN films and the resistivity of SLs remains lower up to temperatures of about 350 °C. For SLs grown on AlGaN underlayers the gain in resistivity is much more moderate and certain advantages in using such SLs are envisaged only for temperatures below room temperature. The reason for this lower gain is a considerable decrease in hole mobility compared to p-GaN. The effect is somewhat tentatively attributed to worse crystalline perfection of these SLs. It is also shown that such SLs are characterized by a strongly broadened MCL peak and the presence of additional hole traps with activation energy of about 0.4 eV. Despite that, the photosensitivity and MCL intensity of these SLs are much better than for reference p-GaN samples.

Deep centers spectra and scanning electron microscope studies of p-GaN films prepared by metallorganic chemical vapor deposition on sapphire

Spectra of deep centers were studied in p-GaN(Mg) films prepared by metallorganic chemical vapor deposition on sapphire. In addition to the E v+0.16 eV Mg acceptor levels hole traps with a level near E v+0.4 eV were observed. Persistent photoconductivity measurements in p-GaN(Mg) films suggest that the phenomenon is very likely due to centers other than Mg. Studies of nonuniformity of recombination properties revealed the presence of high density of defects with characteristic dimensions of several microns for which the density of nonradiative recombination defects was lower than in the matrix. It was also shown that the regions adjacent to pores in p-GaN films show a higher luminescence intensity and a higher magnitude of the electron beam induced current.

Studies of deep centers in high-resistivity p-GaN films doped with Zn and grown on SiC by hydride vapor phase epitaxy

The results of deep levels studies in high-resistivity p-GaN films doped with Zn and grown by hydride vapor phase epitaxy on SiC are presented. These studies show the presence of deep traps with apparent activation energies of 0.4, 0.6 and 0.9 eV. Conductivity versus temperature measurements suggest that the Fermi level in the films is pinned near E v+0.9 eV. From the 0.4, 0.6 and 0.9 eV traps the 0.4 and the 0.9 eV traps are argued to be hole traps in the lower half of the bandgap while the 0.6 eV traps are quite similar to the well known electron traps in the upper half of the bandgap. Microcathodoluminescence (MCL) spectra measurements show the presence of band edge luminescence and of two defect bands at 2.2 and 2.8 eV (the yellow band and the blue band). MCL imaging of the samples indicated also a formation of a network of bright line defects presumably near the GaN/SiC interface.

Studies of defects in Mg doped p-GaN films grown by hydride vapor phase epitaxy on SiC substrates

Deep and shallow levels spectra, microcathodoluminescence (MCL) and electron beam induced current (EBIC) measurements on Mg doped GaN films grown by hydride vapor phase epitaxy directly on SiC are reported. It is shown that the samples contain Mg acceptors with a level at E v+0.18 eV, some unidentified shallow traps, unidentified traps with apparent activation energy of 0.25 eV, both somewhat arbitrarily attributed to interface states, and hole traps with levels at E v+0.4 and E v+0.85 eV. The latter two seem to be related to the blue and yellow luminescence bands observed in the MCL spectra. MCL and EBIC imaging of the samples reveals the presence of the network of misfit dislocations near the GaN/SiC interface. The MCL band-edge band is blue shifted in the vicinity of these defects, most likely because of the strain release near dislocations.

Co-doping Characteristics of Si and Zn with Mg in P-type GaN

We investigated the doping characteristics of Mg doped, Mg-Si co-doped, and Mg-Zn co-doped GaN films grown by metalorganic chemical vapor deposition. We have grown p-GaN film with a resistivity of 1.26 Ωcm and a hole density of 4.3 × 1017 cm−3 by means of Mg-Si co-doping technique. The Mg-Si co-doping characteristic was also explained effectively by taking advantage of the concept of competitive adsorption between Mg and Si during the growth. For Mg-Zn co-doping, p-GaN showing a low electrical resistivity (0.7 Ωcm) and a high hole concentration (8.5 × 1017 cm−3) was successfully grown without the degradation of structural quality of the film. Besides, the measured specific contact resistance for Mg-Zn co-doped GaN film is 5.0 × 10−4 cm2, which is lower value by one order of magnitude than that for only Mg doped GaN film (1.9 × 10−3Ωcm2).

Electrical Properties of the Si Implantation in Mg Doped p-GaN

This work performs Si ion implantation to activate and convert the electrical conduction of p-GaN films from p-type to n-type. Multiple implantation method is used to form a uniform Si implanted region in the p-type GaN epitaxial layer. Implantation energies for the multiple implantation are 40, 100, and 200 keV. The implantation dose is 5 × 1015 cm2 for each implantation energy. After implantation, the samples were annealed in an N2 ambient for different annealing temperatures and annealing times. The activation efficiency reaches as high as 20% when annealing the sample at 1000 °C. The carrier activation energy is about 720 meV. The low activation energy indicates that the hopping process mechanism is the dominant mechanism for the activation of the Si implant in p-GaN. In addition, the rectifying I–V characteristic of the p–n GaN diode is also examined.

Microstructural investigation of oxidized Ni/Au ohmic contact to p-type GaN

The microstructure of oxidized Ni/Au films on p-GaN was examined to elucidate the formation of a low resistance ohmic contact to p-GaN with a field-emission gun transmission electron microscope in conjunction with composition analyses. The p-GaN/Ni/Au samples were heat treated at 500 °C in air mainly composed of a mixture of crystalline NiO, Au, and amorphous Ni–Ga–O phases. Small voids adjacent to the p-GaN film were also observed. The as-deposited Au film converted into discontinuous islands containing small amounts of Ni that connect with p-GaN. NiO formed a continuous film at the surface that covers the Au islands and the amorphous Ni–Ga–O phases. Moreover, NiO partially contacts p-GaN as well as Au islands and the amorphous Ni–Ga–O phase. The orientation relationship of the crystalline NiO, Au-rich islands, and p-GaN film was identified as NiO(111)//Au(111̄)//GaN(0002) and NiO[11̄0]//Au[11̄0]//GaN[112̄0]. The results suggested that Ni atoms diffuse through the Au layer onto the surface and react with oxygen to form NiO, whereas Au atoms diffuse towards the inside to form a Au–Ni alloy. The microstructural examination indicated that the crystalline NiO and/or the amorphous Ni–Ga–O phases may significantly affect the low resistance ohmic contact to p-GaN.

NiIn as an Ohmic Contact to P-GaN

Based on the criteria for the solid state exchange reaction with p-GaN, we have investigated the intermetallic compound NiIn as a possible ohmic contact. The contacts were fabricated by depositing NiIn on p-GaN films (p ∼ 2 × 1017 cm−3) using RF sputtering from a compound target. The as-deposited, NiIn contacts were found to be rectifying and using I-V characterization a Schottky barrier height of 0.82 eV was measured. Rapid thermal annealing of the contacts was shown to significantly decrease their resistance, with contacts annealed at 800 °C for 1 min yielding the lowest resistance. When annealed at 800 °C for 1 min NiIn contacts exhibited a specific contact resistance of 8-9 × 10−3 Ω cm2 as measured by the circular transmission line model. To allow a more universal comparison the more traditional Ni/Au contacts, processed under the same conditions, were used as a standard. Their measured specific contact resistance (ρc = 1.2 − 2.1 × 10−2 Ωcm2) was significantly higher than that of the NiIn contacts. Demonstrating that NiIn has promise as an ohmic contact to p-GaN and should be studied in greater detail.

Co-Doping Characteristics of Si and Zn with Mg in P-Type GaN

AbstractWe investigated the doping characteristics of Mg doped, Mg-Si co-doped, and Mg-Zn codoped GaN films grown by metalorganic chemical vapor deposition. We have grown p-GaN film with a resistivity of 1.26 Ωm and a hole density of 4.3 × 1017 cm−3 by means of Mg-Si co-doping technique. The Mg-Si co-doping characteristic was also explained effectively by taking advantage of the concept of competitive adsorption between Mg and Si during the growth. For Mg-Zn co-doping, p-GaN showing a low electrical resistivity (0.7 Ωm) and a high hole concentration (8.5 × 1017 cm−3) was successfully grown without the degradation of structural quality of the film. Besides, the measured specific contact resistance for Mg-Zn co-doped GaN film is 5.0 × 10−4 cm2, which is lower value by one order of magnitude than that for only Mg doped GaN film (1.9 × 10−3 Ωm2).

Improved Mg-doped GaN films grown over a multilayered buffer

Mg-doped p-GaN films have been grown on sapphire substrate with a multilayered buffer (MLB) by metalorganic chemical vapor deposition and characterized by photoluminescence spectroscopy. The MLB consists of GaN/AlxGa1−xN (0<x⩽1) superlattice-like layer structures although the material combination as well as the periodicity and uniformity in layer thickness and composition are not essential issues for constituting a MLB. It was found that the p-GaN films grown on MLBs gave much stronger optical emission than those grown on conventional GaN or AlxGa1−xN (0<x⩽1) single-layer buffer or GaN/AlxGa1−xN double-layer buffer under identical reactor configuration. This fact indicates that, by using the newly-developed MLB, the crystalline quality of group-III nitride-based-semiconductor epitaxial layers can be significantly improved.

NIIN As an Ohmic Contact to P-GAN

AbstractBased on the criteria for the solid state exchange reaction with p-GaN, we have investigated the intermetallic compound Niln as a possible ohmic contact. The contacts were fabricated by depositing Niln on p-GaN films (p ∼ 2 × 1017 cm-3) using RF sputtering from a compound target. The as-deposited, Niun contacts were found to be rectifying and using I-V characterization a Schottky barrier height of 0.82 eV was measured. Rapid thermal annealing of the contacts was shown to significantly decrease their resistance, with contacts annealed at 800°C for I min yielding the lowest resistance. When annealed at 800°C for 1 min Niln contacts exhibited a specific contact resistance of 8-9 × 10-3 Ωcm2, as measured by the circular transmission line model. To allow a more universal comparison the more traditional Ni/Au contacts, processed under the same conditions, were used as a standard. Their measured specific contact resistance (ρc = 1.2 - 2.1 x 10-2 Ωcm2) was significantly higher than that of the Niln contacts. Demonstrating that Niln has promise as an ohmic contact to p-GaN and should be studied in greater detail.

On the incorporation of Mg and the role of oxygen, silicon, and hydrogen in GaN prepared by reactive molecular beam epitaxy

Mg-doped GaN samples prepared by reactive molecular beam epitaxy have been investigated in an attempt to gain insight into the impurity incorporation and the origin of auto doping in otherwise undoped GaN films. The Hall and secondary ion mass spectroscopy data were utilized for the analysis of possible background impurities such as Si, O, and H in an effort to ascertain whether the background electron concentration is of impurity origin or native defect origin. The data appear to support the N vacancy as a possible cause of auto-n-type doping seen in undoped GaN. The effect of the ammonia flow rate on the incorporation of Mg atoms in GaN films and on the behavior of H were studied for layers grown on c-plane sapphire as well as 6H–SiC. Increased incorporation of Mg with larger ammonia flow rates is attributed to Ga vacancies and accompanying site selection. Moreover, p-GaN films grown under high ammonia flux are reported with a hole concentration, mobility, and resistivity of about 8×1017 cm−3, 26 cm2/V s, and 0.3 Ω cm, respectively.