Variable Temperature Hall Measurements Research Articles

Room temperature sputtered Cu doped NiO1+δ: p-type conductivity, stability of electrical properties and p-n heterojunction

Abstract Nickel oxide is one of the few wide gap p-type materials which has been applied in many optoelectronic devices. It is well known that the p-type conductivity in NiO arises from Ni vacancy VNi (and/or Oxygen interstitial) acceptors in Oxygen-rich NiO (NiO1+δ). However, due to the instability of these native defects, NiO1+δ is unstable, and its p-type conductivity degrades gradually over time, even at room temperature. In this work, we investigate the effects of copper doping on the optoelectronic properties and the stability of the electrical properties of NiO1+δ (CuxNi1-xO1+δ) synthesized at room temperature by magnetron sputtering. Our results show that with up to 8% Cu doping, the p-type resistivity of NiO1+δ decreases almost by an order of magnitude from 3.3 Ω-cm to 0.4 Ω-cm. Variable temperature Hall measurements show that the hole conduction mechanism can be described by small polaron hopping (SPH) conduction. X-ray photoemission spectroscopy (XPS) confirms that Cu is incorporated as Cu+ acceptors in NiO1+δ. Since the p-type conductivity of CuxNi1-xO1+δ films come from both VNi and the more stable Cu acceptors, the electrical properties of these films are found to be more thermally stable compared to NiO1+δ. P-type CuxNi1-xO1+δ films with x = 0.08 exhibit a reasonable resistivity of ∼30 Ω-cm with a transmittance of 60% after annealing at 400 °C. Furthermore, a p-CuxNi1-xO1+δ and an n-type ZnO p-n heterojunction was fabricated, and the structure shows good rectification. XPS reveals that the p-CuxNi1-xO1+δ/n-ZnO heterojunction has a type-II band alignment with a valence band and conduction band offsets of 1.61 ± 0.1 eV and 1.8 ± 0.1 eV, respectively.

Critical increase in Na-doping facilitates acceptor band movements that yields ~180\u2009meV shallow hole conduction in ZnO bulk crystals

Stable p-type conduction in ZnO has been a long time obstacle in utilizing its full potential such as in opto-electronic devices. We designed a unique experimental set-up in the laboratory for high Na-doping by thermal diffusion in the bulk ZnO single crystals. SIMS measurement shows that Na concentration increases by 3 orders of magnitude, to ~3 × 1020 cm−3 as doping temperature increases to 1200 °C. Electronic infrared absorption was measured for Na-acceptors. Absorption bands were observed near (0.20–0.24) eV. Absorption bands blue shifted by 0.04 eV when doped at 1200 °C giving rise to shallow acceptor level. NaZn band movements as a function of doping temperature are also seen in Photoluminescence emission (PL), Photoluminescence excitation (PLE) and UV-Vis transmission measurements. Variable temperature Hall measurements show stable p-type conduction with hole binding energy ~0.18 eV in ZnO samples that were Na-doped at 1200 °C.

Transport behavior of holes in boron delta-doped diamond structures

Boron delta-doped diamond structures have been synthesized using microwave plasma chemical vapor deposition and fabricated into FET and gated Hall bar devices for assessment of the electrical characteristics. A detailed study of variable temperature Hall, conductivity, and field-effect mobility measurements was completed. This was supported by Schrödinger-Poisson and relaxation time calculations based upon application of Fermi's golden rule. A two carrier-type model was developed with an activation energy of ∼0.2 eV between the delta layer lowest subband with mobility ∼1 cm2/Vs and the bulk valence band with high mobility. This new understanding of the transport of holes in such boron delta-doped structures has shown that although Hall mobility as high as 900 cm2/Vs was measured at room temperature, this dramatically overstates the actual useful performance of the device.

Properties of InMnP (0 0 1) grown by MOVPE

We investigated growth and incorporation of Mn into InP (0 0 1) by metal-organic vapour phase epitaxy (MOVPE). Depending on the Mn/In ratio and temperature we found four different incorporation regimes. Flat mirror like layers with high Mn incorporation were either produced around 510 ∘ C or around 600 ∘ C . At higher temperatures or higher Mn-fluxes, the surfaces roughened. We achieved a maximum Mn incorporation around 0.6%, estimated by X-ray diffraction. The corresponding hole concentration was 1.7 × 10 17 cm - 3 . The hole activation energy for the Mn acceptor in variable temperature Hall measurements was 220 meV, comparable to the onset of a broad photoluminescence. Due to this high activation energy the layers showed no spin polarization.

Magneto-Transport Characterization of p-Type HgCdTe

The electrical properties of semiconductor materials have conventionally been extracted via Hall measurements performed at a single magnetic field. When applied to a semiconductor such as HgCdTe with mixed conduction characteristics, the values obtained from the Hall measurement represent only an averaged contribution of all carriers present in the sample. In this study, the transport properties of a liquid-phase epitaxially (LPE) grown p-type HgCdTe were determined. Variable magnetic field and temperature Hall and resistivity measurements were employed in conjunction with the improved-quantitative mobility spectrum analysis (iQMSA) algorithm to extract the concentrations and mobilities of all carriers present in the material. A comparison study was made between a van der Pauw Greek cross and a standard Hall bar structure on the same material. A disparity in the transport property of the sample was observed when both structures were measured within a few days of each other. Through iQMSA analysis, the discrepancy is seen to be attributed to the formation of an n-type skin inversion layer within a week after processing.

Lattice-matched InAlN/GaN two-dimensional electron gas with high mobility and sheet carrier density by plasma-assisted molecular beam epitaxy

We report the growth and electron transport studies of two-dimensional electron gases confined at the lattice-matched In 0.17Al 0.83N/GaN heterostructure on GaN templates by plasma-assisted molecular beam epitaxy. The two-dimensional sheet carrier density, 2.68×10 13 cm −2, at room temperature is the manifestation of spontaneous polarization charge differences between the InAlN and the GaN layers. The heterostructure shows the Hall mobilities of 1080 and 3330 cm 2/V s at 300 and 20 K, respectively. The variable temperature Hall measurements in the range of 20–300 K reveal that the mobility and sheet carrier density have been nearly independent of temperature below about 150 K, a typical behavior of 2DEG structures. The achievement of high mobility can be attributed to the improvement of InAlN epitaxial growth conditions and reduced alloy disorder scattering of carriers.

Influence of AlGaN/GaN interface polarization fields on the properties of photoconductive detectors

The Al0.22Ga0.78N/GaN/Al0.22Ga0.78N multilayer heterostructure photoconductive detectors were designed and fabricated. The influence of AlGaN/GaN interface polarization fields on the properties of the photodetector was investigated. The energy band profile of the heterostructure was approximately calculated. Results indicate that electron and hole wells exist on the GaN sides of the Al0.22Ga0.78N/GaN/Al0.22Ga0.78N interfaces. The existence of two-dimensional electron gas was proved by variable temperature Hall measurements. The spectral response shows that the responsivity of the heterostructure photodetector was as high as 4300 A/W at 355 nm under 3 V bias, which is nearly ten times higher than that of a GaN monolayer structure photodetector. The response time and dark resistance of the photodetectors were also discussed.

Electron transport properties of MM-HEMT with varied channel indium contents

Transport properties of two_dimensional electron gas (2DEG) are crucial to metamorphic high_electron_mobility transistors (MM_HEMT). We have investigated the variations of subband electron mobility and concentration versus temperature from Shubnikov_de Hass oscillations and variable temperature Hall measurements. The results indicate that the electrical performance is the best when the In content is 0.65 in the channel for MM_HEMT. When the In content exceeds 0.65, a large lattice mismatch will cause dislocations and result in the decrease of mobility and the fall of performance in materials and devices.

Epitaxial growth of n-type SiC using phosphine and nitrogen as the precursors

Epitaxial growth of n-type SiC was carried out using PH3 and N2 as the dopant precursors in a chemical vapor deposition system. Thermodynamic simulations were performed to analyze the gas phase composition at the growth conditions. It was shown that the incorporation behaviors of these two dopants are quite different. Compared to nitrogen, phosphorous incorporation has a weaker dependence on flow rate and has a limited site competition effect. Also, phosphorus incorporation for a given precursor flow decreases with temperature while that of nitrogen increases. It was also observed that phosphorous incorporation increases with decreasing C/Si when the ratio is between 0.8 and 3, which suggests phosphorous might incorporate into C site when C/Si is low. Variable temperature Hall measurements were used to characterize the doped layers. Two donor levels at 82–83 and 105–114 meV for phosphorous in 6H–SiC, 50–52 meV, and 78–80 meV for phosphorous in 4H–SiC were resolved. These results were compared to those from ion-implanted samples.

Comprehensive characterization of hydride VPE grown GaN layers and templates

GaN community has recently recognized that it is imperative that the extended, and point defects in GaN and related materials, and the mechanisms for their formation are understood. This is a first and an important step, which must be followed by defect reduction before full implementation of this material and its allied binaries/ternaries in devices. This review is based on a recent concerted effort to establish benchmarks as far as defects are concerned, and identify the basic issues involved. Samples were analyzed for extended defects by TEM and chemical etches, for polarity by electric force microscopy and convergent beam electron diffraction (CBED), for point defects by DLTS, for optical quality and deep defects by photoluminescence (PL), for vacancies by positron annihilation, for donor and acceptor like states within the gap by ODMR and EPR, and for carrier transport targeted for defects and impurities by variable temperature and magnetic field-dependent Hall measurements. Hydride VPE samples grown at Lincoln Laboratories with 1.5, 5.5 and 55 μm thicknesses were investigated for defects by TEM, and their polarity was found to be Ga-polarity, as expected, by CBDE combined with simulations. The density of misfit dislocations at the substrate/EPI interface was determined to be on the order of 10 13 cm -2 based on high-resolution electron microscopy images. The threading dislocation density decreased gradually with distance from the interface, reaching a value of about 10 8 cm -2 at the surface of a 55 μm film. A 200 μm thick laser separated and free-standing HVPE grown GaN template grown at Samsung was also characterized similarly. The free surface and substrate sides were confirmed to be Ga- and N-polarity, respectively. The density of dislocations near the N-face was determined to be, in order, (3 ± 1) x 10 7 and (4 ± 1) x 10 7 by cross-sectional TEM and plan-view TEM, respectively. Identical observations on the Ga-face revealed the defect concentration to be less than 1 x 10 7 cm -2 by plan-view TEM and 5 x 10 5 cm -2 by cross-sectional TEM. Defects in a 10 μm thick GaN layer grown by HVPE at Lincoln Laboratory have been investigated by photo electrochemical (PEC) etching, and by wet etching in hot H 3 PO 4 acid and molten KOH. Threading vertical wires (i.e. whiskers) and hexagonal-shaped etch pits are formed on the etched sample surfaces by PEC and wet etching, respectively. Using atomic force microscopy, one finds the density of whisker-like features to be 2 x 10 9 cm -2 , the same value found for the etch-pit density on samples etched with both H 3 PO 4 and molten KOH. Values agree well with TEM results. A free standing GaN template has been characterized for its structural and optical properties using X-ray diffraction, defect delineation etch followed by imaging with atomic force microscopy (AFM). The Ga-face and the N-face of the c-plane GaN exhibited a wide variation in terms of the defect density. The defect concentrations on Ga- and N-faces were about 5 x 10 5 cm -2 for the former and about 1 x 10 7 cm -2 for the latter, again in good agreement with TEM results mentioned above. High resolution X-ray rocking curves (omega scans) were measured. The [0 0 2] symmetric and [104] asymmetric peaks in 10 μm thick HVPE films had FWHM values between 5.8 and 7.9 arcmin, and 3.9 and 5.2 arcmin, respectively. The Samsung template investigated had wide diffraction peaks (20.6 and 24 arcmin for [0 0 2] and [1 0 4] diffractions, respectively) on the Ga-face, similar for the N-face, when a 2 mm slit size was used. When the slit size was reduced to 0.02 mm, the Ga- and N-face [0 0 2] peaks reduced to 69 and 160 arcsec. A bowing radius of 1.2 m was calculated to account for increased broadening with wider slits.

Activation of Beryllium-Implanted GaN by Two-Step Annealing

AbstractFor the first time, p-type doping through beryllium implantation in gallium nitride was achieved by using a new annealing process, in which the sample was first annealed in forming gas (12% H2 and 88% N2), followed by annealing in pure nitrogen. Variable temperature Hall measurements showed that sheet hole concentrations of the annealed samples were about 1×1013 cm−2 with low hole mobilities. An ionization energy of 127 meV was estimated with a corresponding activation efficiency of ∼ 100%. SIMS results revealed a relationship between the enhanced diffusion of Be and activation of the acceptors.

Activation of Beryllium-Implanted GaN by Two-Step Annealing

AbstractFor the first time, p-type doping through beryllium implantation in gallium nitride was achieved by using a new annealing process, in which the sample was first annealed in forming gas (12% H2 and 88% N2), followed by annealing in pure nitrogen. Variable temperature Hall measurements showed that sheet hole concentrations of the annealed samples were about 1×1013 cm−2 with low hole mobilities. An ionization energy of 127 meV was estimated with a corresponding activation efficiency of ∼ 100%. SIMS results revealed a relationship between the enhanced diffusion of Be and activation of the acceptors.

The growth of an AlGaN/GaN modulation-doped heterostructure by NH 3 source molecular beam epitaxy

Using NH 3 cracked on the growing surface as the nitrogen precursor, an AlGaN/GaN modulation-doped (MD) heterostructure without a buffer layer was grown on a nitridated sapphire substrate in a home-made molecular beam epitaxy (MBE) system. Though the Al composition is as low as 0.036, as deduced from photoluminescence (PL) measurements, the AlGaN barrier layer can be an efficient carrier supplier for the formation of a two-dimensional electron gas (2DEG) at the heterointerface. The 2DEG characteristics are verified by the variable temperature Hall measurements down to 7 K. Using a parallel conduction model, we estimate the actual mobility of the 2DEG to be 1100 cm 2/V s as the sheet carrier density to be 1.0×10 12 cm −2. Our results show that the AlGaN/GaN system is very suitable for the fabrication of high electron mobility transistors (HEMTs).

MBE growth and characterization of in situ arsenic doped HgCdTe

We report the results of in situ arsenic doping by molecular beam epitaxy using an elemental arsenic source. Single Hg1−xCdxTe layers of x ∼0.3 were grown at a lower growth temperature of 175°C to increase the arsenic incorporation into the layers. Layers grown at 175°C have shown typical etch pit densities of 2E6 with achievable densities as low as 7E4cm−2. Void defect densities can routinely be achieved at levels below 1000 cm−2. Double crystal x-ray diffraction rocking curves exhibit typical full width at half-maximum values of 23 arcsec indicating high structural quality. Arsenic incorporation into the HgCdTe layers was confirmed using secondary ion mass spectrometry. Isothermal annealing of HgCdTe:As layers at temperatures of either 436 or 300°C results in activation of the arsenic at concentrations ranging from 2E16 to 2E18 cm−3. Theoretical fits to variable temperature Hall measurements indicate that layers are not compensated, with near 100% activation after isothermal anneals at 436 or 300°C. Arsenic activation energies and 77K minority carrier lifetime measurements are consistent with published literature values. SIMS analyses of annealed arsenic doping profiles confirm a low arsenic diffusion coefficient.

Growth and Characterization of Rare-Earth Phosphide/Arsenide Schottky Contacts to GaAs

ABSTRACTThe lack of high-temperature thermodynamically stable contacts has been a limiting factor for III-V compound semiconductor metallization. The instability of metal/semiconductor contacts at higher temperatures arises due to factors such as thermodynamic instability at the interface, lack of chemical inertness especially to oxygen containing ambients and a large lattice mismatch to the substrate. Our studies of two rare-earth compounds, dysprosium phosphide (DyP) and dysprosium arsenide (DyAs), demonstrate their potential to address the above problems. The growth and characterization of these two materials and their heterostructures will be presented.Both the DyP and DyAs epilayers were grown using MBD. DyP has an excellent room temperature lattice match to GaAs with a mismatch of about 0.01%, whereas DyAs has a lattice mismatch to GaAs on the order of about 2.4%. Consistent high quality DyP and good quality DyAs epilayers, as characterized by TEM, XRD, AES and AFM were obtained for growth temperatures between 500°C and 600°C. The growth rate was about 0.5 μm/hr and and the RMS roughness of the epilayer surface was typically about 0.5 nm and 1.3 nm for Dyp and DyAs, respectively.Electrical characterization of DyP and DyAs include variable temperature Hall measurements, four point probe, TLM, I-V and C-V measurements. Results show that both DyP and DyAs epilayers are n-type with electron concentration between 3–4 × 1020cm−3 and 2–3×1021cm−3, respectively. The room temperature mobility and resistivity of DyP are 300 cm2/Vsec and 60 μ Ω cm, respectively. The room temperature mobility of DyAs is about 50 cm2/Vsec. DyP forms a Schottky barrier to GaAs with a barrier height of 0.81eV and DyAs forms a weak Schottkyt barrier to GaAs. All relevant data will be presented along with schemes for using DyP and DyAs as potential interconnects to III-V compound semiconductors.

Optical and transport studies of highly acceptor doped GaAs/AlGaAs quantum wells

We present a theoretical and experimental study of the properties of highly acceptor-doped quantum wells (QWs), with doping levels varying from 9 × 10 9 to 6 × 10 12 cm −2. From variable temperature Hall measurements it is found that the temperature dependence of the mobility differs remarkably from p-type bulk GaAs, as well as n-type QWs. For the moderately doped ( N A < 3 × 10 11cm −2) sample, the mobility is almost constant with temperature up to ∼ 100K, while the 2D hopping conduction dominates in the same temperature range for the degenerate samples. The activation mechanism involved in the investigated structure can be understood in terms of the change of the subband structure due to the central doping. Steady-state photoluminescence (PL) and PL excitation results are compared with theoretical predictions involving exchange and correlation effects for the electron-hole system and their interaction with acceptor ions. We finally report on the survival of the excitons well above the degenerate limit, due to inefficient screening in the 2D case.

Si and Si/P implants in In0.5Ga0.5P and In0.5Al0.5P

Si and Si/P ion implantation doping of In0.5Ga0.5P and In0.5Al0.5P has been studied for several Si and P doses at energies of 90 and 100 keV, respectively. For single Si implants in InGaP a maximum Hall sheet electron concentration of 1.33×1013 cm−2 is achieved for a Si dose of 5×1013 cm−2. When an optimum dose (2.5×1013 cm−2) P coimplant is performed this electron concentration is increased by 65%. The same dose Si implants in InAlP show a maximum effective activation of 3.9% with no P coimplantation and 5.2% with a P-implant dose 1.5 times the silicon dose. The apparent donor ionization energies are estimated from variable temperature Hall measurements to be 2–5 meV for InGaP and ∼80 meV for InAlP. The deeper level in InAlP is attributed to the DX level found in the Al-containing material. The reduction in sheet resistance associated with the use of Si/P coimplantation in InGaP containing heterostructures should contribute to significant performance enhancements in HFETs and lasers.

P-And N-Type Implantation Doping Of GaN With Ca And O

AbstractIII-N photonic devices have made great advances in recent years following the demonstration of doping of GaN p-type with Mg and n-type with Si. However, the deep ionization energy level of Mg in GaN (∼160 meV) limits the ionized of acceptors at room temperature to less than 1.0% of the substitutional Mg. With this in mind, we used ion implantation to characterize the ionization level of Ca in GaN since Ca had been suggested by Strite [1] to be a shallow acceptor in GaN. Ca-implanted GaN converted from n-to-p type after a 1100°C activation anneal. Variable temperature Hall measurements give an ionization level at 169 meV. Although this level is equivalent to that of Mg, Ca-implantation may have advantages (shallower projected range and less straggle for a given energy) than Mg for electronic devices. In particular, we report the first GaN device using ion implantation doping. This is a GaN junction field effect transistor (JFET) which employed Ca-implantation. A 1.7 µm JFET had a transconductance of 7 mS/mm, a saturation current at 0 V gate bias of 33 mA/mm, a ft of 2.7 GHz, and a fmax of 9.4 GHz. 0-implantation was also studied and shown to create a shallow donor level (∼25 meV) that is similar to Si. SIMS profiles of as-implanted and annealed samples showed no measurable redistribution of either Ca or0inGaNat 1125°C.

Galvanomagnetic properties of two-dimensional electron gases in strain relaxed InxGa1−xAs(x&lt;0.40) heterostructures grown by molecular beam epitaxy on GaAs substrates

We have investigated, as a function of indium content x, the galvanomagnetic and Shubnikov de Haas (SdH) properties of two-dimensional electron gases (2DEG) formed at lattice matched, strain relaxed InAlAs/InGaAs heterojunctions. These were grown by molecular beam epitaxy on GaAs misoriented substrates with a two degree offcut toward the nearest (110) plane. Variable temperature resistivity and Hall measurements indicate an increase in the electron sheet density ns from 0.78×1012cm−2 for x=0.15 to 1.80×1012 cm−2 for x=0.40 at 300K, and from 0.75×1012cm−2 to 1.67×1012cm−2 at T=1.6K. The room temperature electron mobility, measured along the in plane [110], direction is independent of indium content and equals approximately 9500 cm2/Vs. For T<50K, the mobility is independent of temperature decreasing with increasing x from 82000 cm2/Vs for x=0.15 to 33000 cm2/Vs for x=0.40. The ratios (τt/τq) at 1.6K between the electron relaxation time τt and the single particle relaxation time τq, for the strain relaxed specimens, as well as for pseudomorphically strained Al0.35Ga0.65As/In0.15Ga0.85As structures grown on GaAs substrates, and In0.52Al0.48As/In0.53Ga0.47As heterostructures grown lattice matched on InP substrates. Such a study indicates the presence of inhomogeneities in the 2DEGs of the strain relaxed specimens which appear to be related to the process of strain relaxation. Such inhomogeneities, however, have little effect on the electron relaxation time τt which, at low temperatures, is limited principally by alloy scattering.

Degradation of mobility in neutron irradiated GaAs by the increased scattering from multiply charged ionized defects

Mobility changes were measured at temperatures from 15 to 305 K in n-GaAs van der Pauw samples irradiated by fast reactor neutrons. The inverse mobility values, obtained from the variable temperature Hall measurements were fitted using the relation μ−1=AT−3/2+BT3/2. The inverse mobility was found to increase as a result of neutron irradiations over the whole range of temperature; the increase is attributed to the increased scattering from neutron induced charged defects. The values of A found by the least square fitting were used to estimate the increased scattering effect from neutron-induced ionized defects after each step of irradiation. It is concluded that in order to explain the experimental results presented here, the creation of multiply charged defects must be considered.