Kuan-Ting Liu

Magnesium/nitrogen and beryllium/nitrogen coimplantation into GaN

The nitrogen coimplantation characteristics in the Mg- and Be-implanted GaN with different dopant concentration ratios have been systematically investigated. The Hall-effect measurements show that the p-type characteristics are produced in the Mg- and Be-implanted GaN by the coimplantation of N atoms and subsequent annealing, which is essentially related to the column II/V dopant concentration ratio and annealing condition. This behavior may be attributed to the reduction of self-compensation induced by N vacancies and the enhanced acceptor substitution, which is in reasonable agreement with the surface stoichiometric switching determined by x-ray photoelectron spectroscopy measurements. From photoluminescence data, the activation energy of the Be acceptor level is evaluated to be about 145meV, which is shallower than that of the Mg acceptor. These experimental results indicate that the selective-area N coimplantation with Mg and Be atoms into GaN is an effective method to enhance the p-type conductivity ...

Memory and electrical properties of (100)-oriented AlN thin films prepared by radio frequency magnetron sputtering

The (100)-oriented aluminum nitride (AlN) thin films were well deposited onto p-type Si substrate by radio frequency (RF) magnetron sputtering method. The optimal deposition parameters were the RF power of 350 W, chamber pressure of 9 mTorr, and nitrogen concentration of 50%. Regarding the physical properties, the microstructure of as-deposited (002)- and (100)-oriented AlN thin films were obtained and compared by XRD patterns and TEM images. For electrical properties analysis, we found that the memory windows of (100)-oriented AlN thin films are better than those of (002)-oriented thin films. Besides, the interface and interaction between the silicon and (100)-oriented AlN thin films was serious important problem. Finally, the current transport models of the as-deposited and annealed (100)-oriented AlN thin films were also discussed. From the results, we suggested and investigated that large memory window of the annealed (100)-oriented AlN thin films was induced by many dipoles and large electric field applied.

Crystal Polarity Effects on Magnesium Implantation into GaN Layer

Crystal polarity effects on Mg implantation into GaN layers for p-type doping have been systematically investigated. It is found that we can observe a smaller X-ray diffraction full-width at half-maximum and a stronger Mg-acceptor bound exciton emission for the Mg implantation into N-polarity GaN layer than Ga-polarity one after a proper post-implantation annealing treatment. Raman experiment demonstrates that the tensile stress occurs on the as-grown N-polarity GaN layer, which can be resulting from the Ga vacancy. Hall measurement results further indicate that the p-type conductivity can be successfully obtained for N-polarity GaN compared with Ga-polarity one after Mg implantation regardless of under the identical implantation and post-implantation annealing conditions. These phenomena can all be attributed to the more Ga vacancies in the as-grown N-polarity GaN layer that enhances Mg acceptor substitution and eventually achieve p-type conductive characteristics by an appropriate postimplantation annealing treatment.

Electrical and surface composition properties of phosphorus implantation in Mg-doped GaN

Changes in electrical as well as surface composition such as chemical and electronic properties of Mg-doped p-type GaN by phosphorus implanting are systematically investigated using Hall effect and X-ray photoelectron spectroscopy (XPS) measurements. It is shown that p-type conductivity of Mg-doped GaN can be improved by implanting P atoms after a proper post-implantation annealing treatment, probably due to the reduction of self-compensation by P atoms substitution on N vacancy sites. XPS analysis is further found that the decrease of surface oxides and the shift of the surface Fermi level toward the valence band edge through P atoms introduced. These experimental results indicate that the P implantation is an effective method to improve p-type conductivity of Mg-doped GaN and reducing the surface barrier height, which can lead to a lower metal contact resistivity to p-type GaN.

Growth of Be-doped p-type GaN under Invariant Polarity Conditions

We have successfully grown Be-doped p-type GaN on a sapphire (0001) substrate by radio frequency plasma assisted molecular beam epitaxy (RF-MBE) by conserving the surface polarity during growth. It is found that while the surface polarity of undoped GaN grown on a sapphire (0001) substrate is N-terminated, it changes from N- to Ga-terminated during the growth of Be-doped GaN. When this polarity change occurs, the resistivity of the grown layer is extremely high. Moreover, a broad deep level photoluminescence distinctly appears around 2.0 eV. When an AlN buffer layer is used, a GaN layer on this buffer layer can be grown under the Ga-terminated condition. By growing Be-doped GaN on the Ga-terminated GaN, we have successfully grown Be-doped p-type GaN without the polarity change. In addition, the optical property of the sample is markedly improved. We have confirmed that the Be acceptor level is shallower than the Mg acceptor level by approximately 100 meV.

Effects of Phosphorus Implantation on the Activation of Magnesium Doped in GaN

The effects of phosphorus implantation on the activation of magnesium doped in GaN at different dopant concentration ratios have been systematically investigated. Hall effect measurements show that P implantation improves the hole concentration, and that this improvement is dependent on P/Mg dopant concentration ratio and annealing conditions. This phenomenon is attributable to the reduction in self-compensation that results from the formation of deep donors and the enhanced Mg atom activation, which is in reasonable agreement with the optical properties observed by photoluminescence measurements. In addition, a new photoluminescence peak resulting from P-related transitions is also observed, evidently owing to the recombination of electrons from the shallow native donors with holes previously captured by isoelectronic P traps.

Growth of semi-polar (101̄3) AlN films on the silicon

Highly semi-polar (101̄3) oriented and fine structural AlN films were successfully prepared on silicon substrate by rf magnetron sputtering in this research. The dependence of the nitrogen concentrations and the material characteristics of the films (crystalline structure and micro morphology) were investigated. The crystalline structure of the films was determined by X-ray diffraction (XRD) and the surface microstructure of films was quantitatively investigated using an atomic force microscope (AFM). Different nitrogen concentrations (50%, 58%, 67% and 75%) were used to deposit the films. As decreasing the nitrogen concentrations, the XRD intensity of the semi-polar (101̄3) oriented increases, the crystallite size of the films increases and the roughness of top surface decreases. The experimental results demonstrate that the highly semi-polar (101̄3) oriented AlN films appeared at the lower nitrogen concentration.

P-type doping for Be/C co-implantation in GaN

Ion implantation is an enabling technology for selectively doping a large number of elements into certain areas within the sample with a precise control of dopant concentration and depth distribution. Efficient n-type doping in GaN has been proved can be obtained by implantation of Si donor [1]. In the case of p-type doping, where Mg is widely used as an acceptor, a large ionization energy and self-compensation resulting from implantation-induced N vacancies limit the number of holes even with high temperature post-implantation annealing process [1]. So far, only few successes in p-type GaN were achieved by ion implantation technique [1-3]. Beryllium is an attractive choice because of the low damage level it creates in GaN [4] and the theoretical evidence that it has a shallow acceptor level in GaN [5]. However, the problem associated with Be doping is that they tend to stay at interstitial sites (Beint) acting as double donors to produce self-compensation [6]. It has been reported that co-doping with O atoms into Be-doped GaN improved the doping efficiency and established p-type conductivity [3]. On the other hand, Carbon doping in GaN is an interesting topic because it likely becomes a donor when incorporated on the Ga site (CGa), and an acceptor on the N site (CN). It has been predicted that the incorporation of C on N site is preferred [7], but it is accompanied by efficient self-compensation of CN acceptors and CGa donors [8, 9]. Considering that light C atom may introduce much less damage in the lattice than O atom. In this study, we have systematically investigated the Be+C co-implantation characteristics in GaN with different dopant concentration ratios. Co-doping effects such as electrical and optical properties were characterized by Hall and photoluminescence (PL) measurements. The un-doped GaN samples used in this study were all grown by metalorganic chemical vapor deposition (MOCVD) on c-plane sapphire substrates. 2-μm-thick undoped layer was deposited at 1000C right after the growth of a 30 nm low-temperature buffer layer at 550C. After the growth, the un-doped GaN samples were co-implanted with Be+C using a multiple-step implantation technology. Multiple energy Be implants were first performed with total dose of 2.5×10 cm to produce uniform Be concentration of 5×10 cm with a depth of 0.5 μm. Different C doses were then implanted to produce uniform C concentrations corresponding to Be/C ratios of 1 and 2, respectively. After implantation, samples were annealed in N2 ambient separately at 1100C and 1200C for 10s capped with an un-doped GaN wafer in a face-to-face geometry. The as-grown GaN samples were high resistivity with the background electron concentrations of 3~5×10 cm at room temperature. Table 1 shows the electrical characteristics for a series of the implanted samples. Beor C-implanted samples after post-annealing remained n-type conductivity, which could be attributed to the insufficient removal of implantation-induced N vacancies and Beint or CGa donors overcompensated the acceptors. The similar examples were also observed by some research groups for their Beor C-implanted GaN experiments [10, 11]. Compared with individual Beand C-implantation, the Be+C co-implantation converted to p-type conductive characteristics for the sample co-implanted with a relatively lower C dose (Be/C = 2 sample) and 1100C post-annealing. This result seemed reasonable since more defects tend to be introduced by the higher C dose implantation (Be/C = 1 sample) that overcompensate the activated acceptors. Besides, the excessive annealing treatments could lead to dopants out-diffusion resulting in high resistivity for Be/C = 2 co-implanted sample annealed at 1200C, in spite of the identical Be/C dose ratios for both samples. The origin of the improved activation efficiency can be considered into two possibilities. One is the C atom probably acts as a donor in our samples, so that co-doping with CGa donors and Be acceptors will increase the interaction of both types of impurity which can result in a reduction of impurity energy level and an increase in the state density near the band edge. Another possibility is that the implanting additional C atoms occupy on N vacancy sites caused by Be-implantation and enhance both Be and CN acceptors substitutions. The outcomes of which would increase the activation efficiency and solubility of impurities, and eventually achieve p-type conductive characteristics for the Be/C = 2 co-implanted sample after an appropriate annealing treatment. Figure 1 shows 10K PL spectra of Be, C and Be/C = 2 implanted samples after annealing at 1100C. An un-doped GaN sample was also plotted for comparison. The PL spectrum of un-doped sample have shown a strong exciton bound to neutral donor peak (I2) line at 3.47 eV and donor-acceptor pair (DAP) transitions caused by the contamination of C impurity at 3.28 eV followed by two phonon replicas with ~90 meV separation [12]. For the Extended Abstracts of the 2005 International Conference on Solid State Devices and Materials, Kobe, 2005,