First-principles study of nitrogen-mediated Be doping for p-type diamond

Here, we demonstrate chemical potential control as an effective tool to suppress compensation in Mgdoped nitrogen-polar (N-polar) gallium nitride (GaN) films grown by metalorganic chemical vapor deposition (MOCVD). An increase in nitrogen chemical potential obtained by increasing the process supersaturation is shown to systematically reduce the incorporation of compensating oxygen and nitrogen-vacancy point defects. This is confirmed by photoluminescence and temperature-dependent Hall effect measurements. The suppressed compensation led to an order of magnitude improvement in p-type conductivity with the room-temperature hole concentration, mobility, and resistivity measuring 6x10" cm ™~, 9 cm*/Vs, and 1.1 ohm-cm, respectively. These results are paramount in the pathway towards N-polar GaN power and optoelectronic devices. GaN with N-polar orientation has attracted growing interest toward the development of high-performance electronic and optoelectronic devices such as high-frequency high electron mobility transistors (HEMTs), solar cells, longwavelength LEDs, and ultraviolet photocathodes. A reversal of polarization field direction in N-polar GaN from its metal-polar counterpart allows for device designs that lead to improved performance, for example, enhanced carrier extraction efficiency in InGaN solar cells, enhanced hole injection, suppressed electron overflow and decreased droop in LEDs, improved confinement in the two-dimensional electron gas channel in AlGaN/GaN HEMTs, and improved transistor performance in polarization-engineered tunnel field-effect transistors (T-FETs). These novel device applications based on N-polar GaN can be further improved and optimized with the realization of low-resistivity p-type material. For example, achieving a steep device turn-on in T-FETs necessitates low resistance p-GaN layers, whereas high hole and electron concentrations are required near the p-n junction for optimal band alignment in these devices. Achieving efficient p-type doping in GaN is understood to be challenging due to an interplay between large Mg acceptor ionization energy (150-220 meV), incorporation of compensating point defects and point defect complexes during epitaxial growth, and low mobility values of holes as compared to electrons. Moreover, N-polar GaN offers added material challenges that need to be addressed to achieve high p-type conductivity. For instance, if not controlled, N-polar GaN grows with a rough surface morphology characterized by hexagonal hillocks, and high unintentional defect incorporation, notably oxygen, which incorporates at concentrations >10 ~ cm™~. Oxygen on the nitrogen site (O7) acts as a shallow donor in N-polar GaN with an activation energy of ~29 meV which leads to significant difficulties in achieving p-type conductivity. In addition, the incorporation of nitrogen-vacancy (V)))-related compensating defects is favored in p-type GaN due to low formation energy. While much work has focused on pdoping of Ga-polar GaN, to date, comprehensive electrical characterization and compensation control in p-type Npolar GaN is lacking in the literature. The chemical potential control (CPC) is a predictive approach wherein the dependence of the defect formation energy is theoretically estimated as a function of MOCVD growth parameters. These growth parameters are, for example, the V/IIl ratio, diluent gas, temperature, pressure, and input partial pressures of gas species. Instead of these individual MOCVD “knobs”, supersaturation is employed as a representative thermodynamic parameter. Following the CPC framework, increasing the supersaturation can be shown to increase the formation energy of both compensating defects, On and Vn: In this work, we demonstrate the utility of the CPC scheme in reducing the compensation in ptype N-polar GaN to realize state-of-the-art hole concentration and resistivity.

The calculation of charge transition energy level (CTL) and defect formation energy are of significance to explore potential n-type or p-type doping in materials. Based on the first-principles method, this paper systematically studied the structural, magnetic, and defect properties of 12 kinds of dopants in the two-dimensional hexagonal gallium nitride (2D h-GaN) system. The results show that the most stable charge states (MSCSs) for n-type systems are 0 and 1+, and all the n-type substitutes act as shallow donors. The MSCSs of the p-type systems are 1−, 0 and 1+, and the acceptor ionization energy is distributed higher than the valence band maximum (VBM) from ~1.25 to 2.85 eV, acting as deep acceptors, which will capture electrons (holes) in n-(p-type) 2D h-GaN and affect the carrier conductivity. Thus, it is difficult to achieve p-type doping through a single defect in 2D h-GaN, and complex defects are necessary to achieve p-type doping experimentally.

The doping of the wide-band gap semiconductor diamond has led to the invention of many electronic and optoelectronic devices. Impurities can be introduced into diamond during chemical vapor deposition or high pressure-high temperature growth, resulting in materials with unusual physical and chemical properties. For electronic applications one of the main objectives in the doping of diamond is the production of p-type and n-type semiconductors materials; however, the study of dopants in diamond nanoparticles is considered important for use in nanodevices, or as qubits for quantum computing. Such devices require that bonding of dopants in nanodiamond must be positioned substitutionally at a lattice site, and must exhibit minimal or no possibility of diffusion to the nanocrystallite surface. In light of these requirements, a number of computational studies have been undertaken to examine the stability of various dopants in various forms of nanocrystalline diamond. Presented here is a review of some such studies, undertaken using quantum mechanical based simulation methods, to provide an overview of the crystal stability of doped nanodiamond for use in diamondoid nanodevices.

ABSTRACTDiamond has an electric-field breakdown 20 times that of Si and GaAs, and a saturated velocity twice that of Si. This results in a predicted cut off frequency for high-power diamond transistors 40 times that of similar devices made of Si or GaAs. Boron is the only known impurity that can be used to lightly dope diamond. This p-type dopant has an activation energy of 0.3 to 0.4 eV, which results in high-resistivity material that is undesirable for devices. However, heavily boron doped diamond has a very small activation energy and a low resistivity and is of device quality. Transistors can be designed that use only undoped and heavily doped diamond. One of the steps in a device fabrication sequence is homoepitaxial diamond growth. Lightly and heavily doped homoepitaxial diamond films were characterized by scanning and transmission electron microscopy, x-ray diffraction, measurements of resistivity as a function of temperature, and secondary ion mass spectroscopy. It was found that under appropriate growth conditions these films are of device quality.

Ultraviolet photodetectors have many military and commercial applications. However, for many of these applications, the photodetectors must be solar blind. This means that the photodetectors must have a cutoff wavelength of less than about 270 nm. Semiconductor based devices would then need energy gaps of over 4.6 eV. In the AlxGa1-xN system, the aluminum mole fraction, x, required is over 40%. As the energy gap is increased, doping becomes much more difficult, especially p-type doping. This report is a study of the electrical properties of AlxGa1-xN to enable better control of the doping. Magnesium doped p-type AlxGa1- xN has been studied using high-temperature Hall effect measurements. The acceptor ionization energy has been found to increase substantially with the aluminum content. Short-period superlattices consisting of alternating layers of GaN:Mg and AlGaN:Mg were also grown by low-pressure organometallic vapor phase epitaxy. The electrical properties of these superlattices were measured as a function of temperature and compared to conventional AlGaN:Mg layers. It is shown that the optical absorption edge can be shifted to shorter wavelengths while lowering the acceptor ionization energy by using short- period superlattice structures instead of bulk-like AlGaN:Mg. Silicon doped n-type films have also been studied.

The n-type doping of diamond: Present status and pending questions

The utilization of diamond, the ultimate semiconductor, in electronic devices is challenging due to the difficulty of n-type doping. Phosphorus (P)-doped diamond, the most prevalent type of n-type diamond, is still limited by the low solubility of P dopant and undesirable compensating defects such as vacancy defects and hydrogen incorporation. In order to overcome this limitation, strain engineering is introduced to the n-type P-doped diamond theoretically in this work. Uniaxial, equibiaxial, and hydrostatic triaxial strains are applied to the P-doped diamond. The formation energy, charge transition level, defect binding energy and other physical properties of the P-doped diamond are then calculated based on first-principles calculations. The results show that uniaxial, equibiaxial, and hydrostatic triaxial tensile strain can reduce the formation energy and the donor ionization energy of P dopant, and also reduce the binding energy of phosphorus–vacancy (PV) and phosphorus–hydrogen (PH) defects. Our results indicate that under tensile strain, the solubility of the P dopant and the n-type conductivity of the P-doped diamond can be increased, and the formation of compensating defects can be suppressed. Therefore, strain engineering is anticipated to be used to enhance the n-type characteristics of the P-doped diamond, facilitating its application in electronic devices.

Diamond is a highly attractive ultrawide bandgap semiconductor for next-generation high-power switching devices and RF devices for its superior physical and electrical properties. However, the lack of effective n-type dopants in diamond has limited the material to only unipolar p-type device applications. Heterostructure bipolar devices that use better n-type semiconductors together with p-type diamond is an approach to get high performance devices. In this work, p–n–p AlGaAs/GaAs/diamond heterojunction bipolar transistors (HBTs) are proposed and fabricated using a grafting technique. The double-heterojunction is formed by transferring an AlGaAs(p-type)/GaAs(n-type) membrane onto single-crystalline p-type doped diamond with an electron affinity of 0.32 eV. The epitaxial AlGaAs/GaAs emitter-base p–n junction shows an ideality factor of 1.09 with an Ion/Ioff of 1.53 × 107 at ± 1.5 V. The grafted GaAs/diamond n–p junction shows an ideality factor of 3.67 with an Ion/Ioff of 3.74 × 1010 at ± 5.2 V. Due to the valence-band energy barrier of 0.3 eV between the GaAs base and the diamond collector, the measured current gain for the HBT is slightly below unity. Simulations show that by reducing the electron affinity value of the p-type diamond, the base-collector energy barrier height can be correspondingly reduced, and high current gain can be expected.

Prospects of enhancement of p-type conductivity in ZnO nanowires

Electrical, optical and structural properties of p-type single crystal silicon were investigated in this work. Electrical conductivity of p-type silicon was measured in the temperature ranges 190 - 300 K. The acceptor ionization energy (?EA) was between 0.047 - 0.051 eV. Photoconductivity of the material was investigated by varying sample current, light intensity and temperature at a constant chopping frequency of 45.60 Hz. Absorption co-efficient (?) of the material was calculated from optical transmittance and reflectance measurements at room temperature (300 K) in the wavelength range of 300 -2500 nm. The direct optical band gap energy was found between 2.10 - 2.20 eV and the indirect optical band gap energy was found between 0.95 – 1.0 eV. The lattice parameter (a) was found to be 5.419Å from X-ray diffraction method (XRD). DOI: http://dx.doi.org/10.3329/dujs.v63i1.21766 Dhaka Univ. J. Sci. 63(1): 37-41, 2015 (January)

Recent progress in chemical vapour deposition (CVD) diamond technology has enabled the preparation of high-quality n-type CVD diamond layers using phosphorus as a dopant. CVD diamond can therefore be considered as a new interesting conventional wide-gap semiconducting material having both n- and p-type dopants, which makes it attractive for numerous applications in high-temperature, high-voltage and high-frequency devices. The concentration of phosphorus in n-type CVD diamond can be controlled in the concentration range of 1 × 1016–5 × 1019 cm−3 with a carrier mobility exceeding 600 cm2 V−1 s−1. In this review, the most relevant questions concerning the preparation of P-doped diamond are addressed and discussed in terms of future progress and novel electronic devices. We not only discuss the preparation of single crystal epitaxial diamond but also address the growth of large-area n-type polycrystalline CVD diamond, which can be useful for several applications such as for detectors or electron emitters.

Ion implantation and diamond: some recent results on growth and doping

Nitride-based device structures for electronic and optoelectronic applications usually incor-porate layers of AlxGa1−xN, and n- and p-type doping of these alloys is typically required. Experimental results indicate that doping efficiencies in AlxGa1−xN are lower than in GaN. We address the cause of these doping difficulties, based on results from first-principles density-functional-pseudopotential calculations. For n-type doping we will discuss doping with oxygen, the most common unintentional donor, and with silicon. For oxygen, a DX transition occurs which converts the shallow donor into a negatively charged deep level. We present experimental evidence that oxygen is a DX center in AlxGa1−xN for x>∼0.3. For p-type doping, we find that compensation by nitrogen vacancies becomes increasingly important as the Al content is in-creased. We also find that the ionization energy of the Mg acceptor increases with alloy composition x. To address the limitations on p-type doping we have performed a comprehensive investigation of alternative acceptor impurities; none of the candidates exhibits characteristics that surpass those of Mg in all respects.

Nitride-based device structures for electronic and optoelectronic applications usually incorporate layers of AlxGal-xN, and n- and p-type doping of these alloys is typically required. Experimental results indicate that doping efficiencies in AlxGal-xN are lower than in GaN. We address the cause of these doping difficulties, based on results from first-principles density-functional-pseudopotential calculations. For n-type doping we will discuss doping with oxygen, the most common unintentional donor, and with silicon. For oxygen, a DX transition occurs which converts the shallow donor into a negatively charged deep level. We present experimental evidence that oxygen is a DX center in AlxGal-xN for x>∼0.3. For p-type doping, we find that compensation by nitrogen vacancies becomes increasingly important as the Al content is increased. We also find that the ionization energy of the Mg acceptor increases with alloy composition x. To address the limitations on p-type doping we have performed a comprehensive investigation of alternative acceptor impurities; none of the candidates exhibits characteristics that surpass those of Mg in all respects.

Energy-driven multi-step structural phase transition mechanism to achieve high-quality p-type nitrogen-doped β-Ga2O3 films