N-type Single Crystals Research Articles

Enhanced thermoelectric performance in polycrystalline N-type Pr-doped SnSe by hot forging

SnSe has attracted significant attention for use in thermoelectric devices due to its reported ultrahigh figure-of-merit ZT in both p- and n-type single crystals, which inspired us to explore the thermoelectric properties of SnSe polycrystals for potential large-scale applications. Here, n-type Sn1-xPrxSe polycrystals are prepared by ball milling and hot pressing. A maximum ZT value of ~0.7 at 773 K is achieved in Sn0.97Pr0.03Se due to the enhanced electron concentration and electrical conductivity resulting from Pr doping at the Sn site. Through hot forging for a higher degree of orientation, a lowered thermal conductivity is obtained along the hot-pressing direction, contributing to an enhanced ZT value of ~0.9 at 773 K in this direction. This study paves a new way for optimization of n-type SnSe by cation-site lanthanide doping.

Highly conductive n-type CH3NH3PbI3 single crystals doped with bismuth donors

Highly conductive n-type CH3NH3PbI3 single crystals are grown by bismuth doping and the optical fingerprints of bismuth induced donors in CH3NH3PbI3 perovskites are identified.

Delocalized Carriers and the Electrical Transport Properties of n-Type GeSe Crystals

Recent discovery of a high thermoelectric figure of merit in n-type SnSe single crystal stimulates further research on the analogue compounds. In this work, n-type GeSe is predicted to have a significantly higher normalized power factor along the cross-plane direction based on density functional theory (DFT) calculations and Boltzmann transport theory. This high normalized power factor is due to the high normalized electrical conductivity, which originates from the more delocalized interlayer carrier density. Furthermore, the effects of several potential n-type dopants (group VA and group VIIA elements) on the electronic structure and electrical transport properties of GeSe have been studied. We find that the group VA dopants decrease the density of states near the conduction band minimum and result in a decrease of the Seebeck coefficient of GeSe. As a result, n-type GeSe doped with group VIIA elements is predicted to have a higher normalized power factor than that doped with group VA elements.

Progress in Single Crystal Growth of Wide Bandgap Semiconductor SiC

The research and commercialization of SiC based power device have been burgeoning over the last decade worldwide, which is bringing about an increasing demand on lost-cost and low-defect SiC wafers. To meet this challenge, we have been continuously making efforts on improving the crystal growth and wafer processing techniques. Now, the mass-production of high quality 4-inch, 6-inch n-type and semi-insulating SiC wafers has been realized. Statistically, the micropipe density is lower than 0.5 cm-2. The resistivity of the wafers is lower than 0.02 Ω·cm and up to 108 Ω·cm for n-type and semi-insulating SiC single crystals, respectively. A state of the art processing technique has been developed to control wafer deformation and thickness within the desired values for subsequent epitaxy. The total defect number of the epitaxial layers grown on the "epi-ready" 4-inch SiC wafer is 63, and the usable area is 97.6%, indicating the high quality of our SiC substrates.

Feasibility study of the production of bulk stable Ge isotopes by the hydrogen plasma chemical reduction of fluorides

A «fluoride» method for preparation of germanium isotopes was developed and tested, including two main stages: 1) germanium deposition in the process of plasma chemical hydrogen reduction of germanium tetrafluoride with the subsequent compaction of the deposited polycrystalline germanium; 2) zone refining and single grystal growth of germanium if required. Degree of conversion of germanium tetrafluoride of 90–95% was achieved with a productivity of up to 10 g/h. The total yield of about 38% was attained for 72Ge n-type single crystal with a purity of >7 N. The method suggested has been compared with methods described in literature. Its advantage is the high yield production of germanium isotopes directly from enriched tetrafluoride without optional reagents. Plasma-chemical reduction of germanium tetrafluoride does not require sophisticated equipment. Thus, the “fluoride” method could be promising for production of germanium isotopes on industrial scale.

Longitudinal phonon plasmon mode coupling in β -Ga2O3

In this letter, we investigate a set of n-type single crystals of monoclinic symmetry β-Ga2O3 with different free electron concentration values by generalized far infrared and infrared spectroscopic ellipsometry. In excellent agreement with our previous model prediction, we find here by experiment that longitudinal-phonon-plasmon coupled modes are polarized either within the monoclinic plane or perpendicular to the monoclinic plane. As predicted, all modes change the amplitude and frequency with the free electron concentration. The most important observation is that all longitudinal-phonon-plasmon coupled modes polarized within the monoclinic plane continuously change their direction as a function of free electron concentration.

Effect of proton doping and heat treatment on the structure of single crystal silicon

The quality and structural perfection of single crystal silicon have been studied using double-crystal X-ray diffraction after hydrogen ion implantation and thermal annealing used in a number of semiconductor technologies. The fundamental difference of this approach is the possibility to rapidly obtain reliable experimental results which were confirmed using X-ray topography. Data have been presented for the condition of the damaged layer in n-type silicon single crystals (r = 100 W × cm) having the (111) orientation and a thickness of 2 mm after proton implantation at energies E = 200, 300 and 100 + 200 + 300 keV and dose D = 2 × 1016 cm-2 and subsequent heat treatment in the T = 100–900 °C range. Using the method of integral characteristics we have revealed a nonmonotonic dependence of the integral characteristics of the damaged layer, i.e., the mean effective thickness Leff and the mean relative deformation Da/a, on the annealing temperature, the maximum deformation being observed for ~300 °C. The results have allowed us to make a general assessment of the damaged layer condition after heat treatment.

IR photoresponse characteristics of Mg2Ge pn-junction photodiodes fabricated by rapid thermal annealing

In this study, high purity n-type Mg2Ge single crystals are grown in graphite crucibles under nitrogen atmosphere using a facile growth process. X-ray diffraction proves that the grown crystal consists of only Mg2Ge single phase without any other additional impurity peaks. The clear symmetrical Laue diffraction pattern of the cleaved substrate demonstrates the single crystalline nature of the grown Mg2Ge crystal ingot. The Hall Effect measurement of the polished Mg2Ge wafer shows that the latter had a moderate carrier density (5.66 * 1016 cm−3), mobility (253 cm2/V) and electrical resistivity (0.5 Ω. cm) at room temperature. Detailed analysis of the temperature dependence of the carrier density demonstrates that the shallow donor level, which gives rise to the n-type conduction of grown Mg2Ge crystals is due to the unintentional Al impurities from Mg source material. Mg2Ge pn-junction photodiodes are also manufactured for the first time by thermal diffusion of silver thin layer into n-Mg2Ge at 550 °C for 5 min in an argon atmosphere. The results reveal that the fabricated p-n photodiodes had a clear rectifying behavior and a remarkable spectral response in the wavelength range from 0.8 to 1.7 μm with a maximum room temperature zero-biased photoresponse of 7 mA/W at 1.2 μm. These outcomes indicate that the fabricated Mg2Ge photodiodes are promising to detect short-wavelength infrared (SWIR) light in the above-mentioned wavelength domain.

Single-Crystal Growth of Cl-Doped n-Type SnS Using SnCl2 Self-Flux.

SnS is a promising photovoltaic semiconductor owing to its suitable band gap energy and high optical absorption coefficient for highly efficient thin film solar cells. The most significant carnage is demonstration of n-type SnS. In this study, Cl-doped n-type single crystals were grown using SnCl2 self-flux method. The obtained crystal was lamellar, with length and width of a few millimeters and thickness ranging between 28 and 39 μm. X-ray diffraction measurements revealed the single crystals had an orthorhombic unit cell. Since the ionic radii of S2- and Cl- are similar, Cl doping did not result in substantial change in lattice parameter. All the elements were homogeneously distributed on a cleaved surface; the Sn/(S + Cl) ratio was 1.00. The crystal was an n-type degenerate semiconductor with a carrier concentration of ∼3 × 1017 cm-3. Hall mobility at 300 K was 252 cm2 V-1 s-1 and reached 363 cm2 V-1 s-1 at 142 K.

Interface trapping in (2¯01) β-Ga2O3 MOS capacitors with deposited dielectrics

The electrical properties of interfaces and the impact of post-deposition annealing have been investigated in gate oxides formed by low pressure chemical vapor deposition (LPCVD SiO2) and atomic layer deposition (Al2O3) on (2¯01) oriented n-type β-Ga2O3 single crystals. Capacitance-voltage based methods have been used to extract the interface state densities, including densities of slow ‘border’ traps at the dielectric-Ga2O3 interfaces. It was observed that SiO2-β-Ga2O3 has a higher interface and border trap density than the Al2O3-β-Ga2O3. An increase in shallow interface states was also observed at the Al2O3-β-Ga2O3 interface after post-deposition annealing at higher temperature suggesting the high temperature annealing to be detrimental for Al2O3-Ga2O3 interfaces. Among the different dielectrics studied, LPCVD SiO2 was found to have the lowest dielectric leakage and the highest breakdown field, consistent with a higher conduction band-offset. These results are important for the processing of high performance β-Ga2O3 MOS devices as these factors will critically impact channel transport, threshold voltage stability, and device reliability.

Relationship between carbon concentration and carrier lifetime in CZ-Si crystals

This paper aims to clarify the effect of carbon concentration on carrier lifetime in as-grown n-type and non-doped silicon crystals produced via the Czochralski (CZ) method. We grew n-type and non-doped silicon single crystals with 3-in. diameters along with different carbon and phosphorous contents. The resistivity, concentrations of oxygen and carbon, and lifetime were measured using four-point measurements, Fourier-transform infrared spectroscopy, and the eddy current method, respectively. The oxygen concentrations of the crystals were 6–8 × 1017 atoms/cm3, and the bulk lifetimes ranged from 10 to 20 ms. The carrier lifetime of CZ silicon crystals depended on dopant concentration but had no significant dependence on carbon concentration.

Features of low-temperature thermal expansion of n-type Bi2Se3 single crystals in magnetic field

Tensometric study of n-type Bi2Se3 single crystals in dc magnetic fields to 6 T in a temperature range of 7–23 K detected a weak negative thermal expansion (NTE) in the basal plane. The NTE increases with the field strength and depends on its orientation with respect to the trigonal c axis. In a magnetic field of 6 T, parallel to the c axis, the linear NTE coefficient reaches −7 · 10−7 K−1, and a minimum sample length is reached at a temperature of 13 K, where a Hall carrier concentration maximum is also detected. The found magnetoelastic anomaly can be associated with the topological insulator state.

A comparative study of wet etching and contacts on [formula omitted] and (010) oriented β-Ga2O3

We report on the effect of β-Ga2O3 crystal orientation on wet etching and Ohmic contact formation. The photochemical etching rate in KOH solutions of (2¯01) oriented, n-type bulk single crystals grown by the edge-defined film-fed growth method is ∼3–4 times higher than for the (010) planes. The activation energy for etching was 0.498 eV and 0.424 eV for (2¯01) and (010) orientations, respectively, suggesting the etching is reaction-limited with the same rate-limiting step. Ti (200 Å)/Au (1500 Å) metallization deposited on the two different orientations and annealed at 450 °C showed Ohmic current-voltage (I-V) behavior for (2¯01) but rectifying characteristics for (010). For (010) Ga2O3, there exists 2 types of surfaces having Ga and O atomic densities of 0.58 and 0.87 × 1015 cm−2, respectively. By contrast, for (2¯01) Ga2O3 surfaces, there exist 2 types of surface, with each type terminated with only Ga or O. If the surface is terminated with O, the dangling bond densities of O are 1.78 and 2.68 × 1015 cm−2, respectively. We found that (2¯01)–oriented Ga2O3 is etched at higher rates and is easier to form Ohmic contacts than (010) Ga2O3. The higher density of oxygen dangling bonds on the (2¯01) plane correlates with the faster etch rates and pronounced Ohmic behavior from deposited metals.

N-type GaSb single crystals with high below-band gap transmission**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474104 and 61504131).

Te-doped GaSb single crystals are studied by measuring Hall effect, infrared (IR) transmission and photoluminescence (PL) spectra. It is found that the n-type GaSb with IR transmittance can be obtained as high as 60% by the critical control of the Te-doping concentration and electrical compensation. The concentration of the native acceptor-associated defects is apparently low in the Te-doped GaSb compared with those in undoped and heavily Te-doped GaSb. The mechanism for the high IR transmittance is analyzed by considering the defect-involved optical absorption process.

High quality crystal growth and anisotropic physical characterization of β-Ga2O3 single crystals grown by EFG method

β-Ga2O3 is an ultra wide-bandgap semiconductor with promising applications in high voltage and temperature electronic devices and deep-ultraviolet optoelectronic devices, and has attracted lots of attention recently. However, there are still many unknowns about the basic properties needed to be explored urgently, which are very important and necessary for the devices design and application. In this work, bulk β-Ga2O3 single crystals with a width of 1 inch were grown by using an optimized edge-defined film-fed growth (EFG) method under Ar plus 50% CO2 atmosphere. High crystalline quality of the as grown crystal has been demonstrated by the high-resolution X-ray diffraction with 43.2 arcsec of the full width at half maximum (FWHM). The fundamental physical properties of different orientated samples, including thermal properties as a function of temperature up to 500 °C, electrical properties, refractive index and hardness were investigated systematically. Moreover, the measurement showed that aluminum turned out to be a good Ohimc electrode for n-type β-Ga2O3 single crystal.

Efficient positron moderation with a commercial 4H-SiC epitaxial layer

We have studied the properties of a commercially available 4H-SiC epitaxial layer and evaluated its potential application as an efficient positron remoderator. A remoderation efficiency of more than 65% has been measured for incident positrons with 1 keV energy. We have determined the work function and the energy distribution of the emitted slow positrons, a property which is essential for practical applications. Comparison of the positron moderation properties of the epitaxial layer with results from a n-type 4H-SiC single crystal, indicate that the epitaxially grown layer is a superior secondary moderator than its substrate counterpart.

Carrier emission of n-type gallium nitride illuminated by femtosecond laser pulses

The carrier emission efficiency of light emitting diodes is of fundamental importance for many technological applications, including the performance of GaN and other semiconductor photocathodes. We have measured the evolution of the emitted carriers and the associated transient electric field after femtosecond laser excitation of n-type GaN single crystals. These processes were studied using sub-picosecond, ultrashort, electron pulses and explained by means of a “three-layer” analytical model. We find that for pump laser intensities on the order of 1011 W/cm2, the electrons that escaped from the crystal surface have a charge of ∼2.7 pC and a velocity of ∼1.8 μm/ps. The associated transient electrical field evolves at intervals ranging from picoseconds to nanoseconds. These results provide a dynamic perspective on the photoemission properties of semiconductor photocathodes.

Achieving ZT=2.2 with Bi-doped n-type SnSe single crystals.

Recently SnSe, a layered chalcogenide material, has attracted a great deal of attention for its excellent p-type thermoelectric property showing a remarkable ZT value of 2.6 at 923 K. For thermoelectric device applications, it is necessary to have n-type materials with comparable ZT value. Here, we report that n-type SnSe single crystals were successfully synthesized by substituting Bi at Sn sites. In addition, it was found that the carrier concentration increases with Bi content, which has a great influence on the thermoelectric properties of n-type SnSe single crystals. Indeed, we achieved the maximum ZT value of 2.2 along b axis at 733 K in the most highly doped n-type SnSe with a carrier density of −2.1 × 1019 cm−3 at 773 K.

Observation of Charge Separation and Space-Charge Region in Single-Crystal P3HT/C60 Heterojunction Nanowires.

We directly observed charge separation and a space-charge region in an organic single-crystal p-n heterojunction nanowire, by means of scanning photocurrent microscopy. The axial p-n heterojunction nanowire had a well-defined planar junction, consisted of P3HT (p-type) and C60 (n-type) single crystals and was fabricated by means of the recently developed inkjet-assisted nanotransfer printing technique. The depletion region formed at the p-n junction was directly observed by exploring the spatial distribution of photogenerated carriers along the heterojunction nanowire under various applied bias voltages. Our study provides a facile approach toward the precise characterization of charge transport in organic heterojunction systems as well as the design of efficient nanoscale organic optoelectronic devices.

Polaron character of the near-EF band of cleaved In2O3(111) single crystals

The near-EF band of the in situ cleaved (111) surface of high-quality n-type In2O3 single crystals was investigated by high-resolution angle-resolved photoemission (ARPES) along the major symmetry lines of the three-dimensional Brillouin zone. Several criteria to pin down Fermi level crossings and Fermi momenta were applied. The near-EF band is of three-dimensional character connected to the bottom of the conduction band. Since the model of a degenerate semiconductor due to high n-type doping is not sufficient the interaction of electron and phonon degrees of freedom is discussed. It is found that the model of Landau-Pekar polarons is capable of describing the observed lineshape qualitatively and the bandwidth of the occupied part of the conduction band quantitatively.