Low Hole Concentration Research Articles

Impact of roll-over-shaped current–voltage characteristics and device properties of Ag(In,Ga)Se2 solar cells

The roll-over shape often observed in the current–voltage curve of Ag(In,Ga)Se2 (AIGS) solar cells degrades the open circuit voltage (VOC) and particularly the fill factor (FF). The origin of the roll-over shape was investigated by experimental measurements and device simulation. By combining AC Hall measurement and the peel-off process, we estimated the AIGS hole concentration to be 2.2 × 1012 cm−3. Theoretical simulation revealed that the roll-over shape is attributed to this low hole concentration. Under an applied forward bias, the band bending near the back contact of the AIGS layer forms an intrinsic semiconductor owing to the injected electrons, leading to the formation of an inverted diode. To solve this issue, the addition of NaF by the postdeposition treatment of the AIGS layer was performed. As a result, the hole concentration of the AIGS layer increased, significantly improving its VOC, FF, and conversion efficiency.

Collective mode at Lifshitz transition in iron-pnictide superconductors

We obtain the exact low-energy spectrum of two mobile holes in a t-J model for an isolated layer in an iron-pnictide superconductor. The minimum dxz and dyz orbitals per iron atom are included, with no hybridization between the two. After tuning the Hund coupling to a putative quantum critical point (QCP) that separates a commensurate spin-density wave from a hidden-order antiferromagnet at half filling, we find an s-wave hole-pair groundstate and a d-wave hole-pair excited state. Near the QCP, both alternate in sign between hole Fermi surface pockets at the Brillouin zone center and emergent electron Fermi surface pockets at momenta that correspond to commensurate spin-density waves (cSDW). The dependence of the energy splitting with increasing Hund coupling yields evidence for a true QCP in the thermodynamic limit near the putative one, at which the s-wave and d-wave Cooper pairs are degenerate. A collective s-to-d-wave oscillation of the macroscopic superconductor that couples to orthorhombic shear strain is also identified. Its resonant frequency is predicted to collapse to zero at the QCP in the limit of low hole concentration. This implies degeneracy of Cooper pairs with s, d and symmetry in the corresponding quantum critical state. We argue that the critical state describes Cooper pairs in hole-doped iron superconductors at the Lifshitz transition, where electron bands first rise above the Fermi level. We thereby predict that the s-to-d-wave collective mode observed by Raman spectroscopy in Ba1−xKxFe2As2 at optimal doping should also be observed at higher doping near the Lifshitz transition.

Ag-N dual acceptor doped p-type ZnO thin films by DC reactive magnetron co-sputtering

ZnO p-type thin films were deposited by dual acceptor co-doping using nitrogen and silver via DC reactive magnetron co-sputtering. As precursor material were used a Zn and an Ag metallic targets with a purity of 99.99%. X-ray energy dispersive spectroscopy (EDX) confirmed the presence of Ag and N in ZnO: Ag,N films. The electrical properties were explored by Hall Effect measurement and showed a low hole concentration for the as-deposited ZnO: Ag,N film. However, after annealing treatment, the films remained p-type and the electrical properties were improved significantly. The best electrical properties showed a low resistivity of 8.56×10−3Ωcm, Hall mobility of 23cm2/Vs and a very high hole concentration of 3.17×1019cm−3.

Thermally induced native defect transform in annealed GaSb**Project supported by the National Natural Science Foundation of China (Grant Nos. 61474104 and 61504131).

Undoped p-type GaSb single crystals were annealed at 550–600 °C for 100 h in ambient antimony. The annealed GaSb samples were investigated by Hall effect measurement, glow discharge mass spectroscopy (GDMS), infrared (IR) optical transmission and photoluminescence (PL) spectroscopy. Compared with the as-grown GaSb single crystal, the annealed GaSb samples have lower hole concentrations and weak native acceptor related PL peaks, indicating the reduction of the concentration of gallium antisite related native acceptor defects. Consequently, the below gap infrared transmission of the GaSb samples is enhanced after the thermal treatment. The mechanism about the reduction of the native defect concentration and its influence on the material property were discussed.

Low-temperature growth of GaSb epilayers on GaAs (001) by molecular beam epitaxy

Non-intentionally doped GaSb epilayers were grown by molecular beam epitaxy (MBE) on highly mismatched semi-insulating GaAs substrate (001) with 2 offcut towards [110]. The effects of substrate temperature and the Sb/Ga flux ratio on the crystalline quality, surface morphology and electrical properties were investigated by Nomarski optical microscopy, X-ray diffraction (XRD) and Hall measurements, respectively. Besides, differential Hall was used to investigate the hole concentration behaviour along the GaSb epilayer. It is found that the crystal quality, electrical properties and surface morphology are markedly dependent on the growth temperature and the group V/III flux ratio. Under the optimized parameters, we demonstrate a low hole concentration at very low growth temperature. Unfortunately, the layers grown at low temperature are characterized by wide FWHM and low Hall mobility.

Low power deposition of the polycrystalline CuxO film with a high mobility and a low hole concentration by radio-frequency magnetron sputtering of a Cu2O target

Copper oxide (CuxO) thin films were prepared by radio-frequency magnetron sputtering with a Cu2O target. The sputtering power was varied from 40 W to 80 W while the working pressures (WPs) were 3–10 mtorr. Deposition rates, surface morphologies, carrier concentrations, carrier mobilities, optical characteristics, bandgaps and crystallinities of the deposited films were investigated. The preferential orientations of the films were Cu2O(111) and CuO(-111). A higher WP can lead to a significant increase in the Cu2O(111) phase and the size of the grain nanoclusters. A higher sputtering power can exhibit a higher hole mobility and a lower hole concentration. The average transmittances in the visible light range and optical bandgaps of the CuxO films were 33–50% and 2.6–2.7 eV. After a post annealing process at 300 °C for 1 h in air, the nanocluster size of the CuxO film prepared at the power of 80 W with WP of 5 mtorr can be increased from 40 nm to 70 nm. Meanwhile, a high mobility of 81.4 cm2/V s, a low hole concentration of 4.2 × 1013 cm−3, and a low density of states of 5 × 1013 cm−3eV−1 can be simultaneously obtained. Besides, a CuxO/ZnO heterojunction diode showing rectifying characteristic was fabricated to verify the semiconductor characteristic of the deposited p-type CuxO film.

Characterizations of Indium-Doped Cu<sub>2</sub>SnSe<sub>3</sub> Bulks Made by Reactive Sintering at 550 °C

Indium-doped Cu2SnSe3 bulk materials with the Cu2(Sn1-xInx)Se3 (In-x-CTSe) formula at x = 0, 0.05, 0.1, 0.15, and 0.2 were prepared at 550 °C for 2 h with soluble sintering aids of Sb2S3 and Te. Defect chemistry was studied by measuring structural and electrical properties of In-x-Cu2SnSe3 as a function of dopant concentration. In-x-CTSe pellets show p-type at x = 0, 0.05 and 0.1 and n-type at x = 0.15 and 0.2. The low hole concentration of 4.56×1017 cm-3 and high mobility of 410 cm2 V-1 s-1 were obtained for Cu2(Sn1-xInx)Se3 bulks at x= 0.1 (10% In). The explanation based upon the In-to-Cu antisite defect for the changes in electrical property was declared.

Temperature dependence of current gain in 4H-SiC bipolar junction transistors

The temperature dependence of current gain from 140 to 460 K in 4H-SiC bipolar junction transistors (SiC BJTs) was investigated. The current gain increased from 110 to 1200 with decreasing temperature from 460 to 200 K. The high current gain at the low temperature can be ascribed to the enhanced incomplete ionization of aluminum acceptors in the base layer, resulting in the increase in injection efficiency. However, when the temperature was further reduced from 200 to 140 K, the current gain decreased from 1200 to 515, which is caused by high injection condition in the base layer, because of a very low hole concentration below 200 K.

Localized surface plasmon resonances and its related defects in orthorhombic Cu3SnS4 nanocrystals

Orthorhombic Cu3SnS4 nanocrystals (NCs) possessing obvious localized surface plasmon resonances (LSPR) in the near-infrared region (NIR) frequencies are prepared. In Cu3SnS4 NCs, valence state of Cu remains close to +1, indicating Cu+ occupies positions originally belong to Cu2+ in a similar way as Cu+ replaces Zn2+ to form CuZn antisites in Cu2ZnSnS4. The increase of Cu/Sn ratio results in slightly red-shifted and weakened NIR absorption. Moreover, LSPR is also verified in wurtzite Cu2SnS3 NCs, although the observed peak maximum is red-shifted comparing to the LSPR band of Cu3SnS4 NCs. But the calculated free carrier concentration for Cu2SnS3 by Drude model is on the same order of magnitude with Cu3SnS4 in which sulfur vacancies acting as compensation centers could reduce hole density to some extent. Whereas Cu2ZnSnS4 presents almost no absorption due to its relatively lower hole concentration.

Defect chemistry and surface oxygen exchange kinetics of La-doped Sr(Ti,Fe)O3 − α in oxygen-rich atmospheres

The mixed ionic and electronic conductor Sr(Ti,Fe)O3-α (STF) exhibits fast oxygen surface exchange kinetics, with electron concentration potentially playing a key role. The effect of La donor doping on electron concentration, Fermi level, and overall defect chemistry of STF is investigated on Sr1-yLayTi0.65Fe0.35O3-α (LSTF, 0≤y≤0.5) thin films. Defect chemical modeling, optical absorption, and electrical conductivity measurements in oxidizing conditions indicate compensation of donors by an increase in oxygen and electron concentrations, increase in Fermi level, decrease in oxygen vacancy and hole concentrations, and formation of cation vacancies (for [La]>[Fe]). The surface exchange coefficient, measured by impedance spectroscopy, decreased with increasing donor concentration, suggesting that oxygen exchange kinetics in LSTF are limited by low oxygen vacancy and/or hole concentrations, rather than electron transfer.

Sb-Doped S/D Ultrathin Body Ge-On Insulator nMOSFET Fabricated by Improved Ge Condensation Process

We report the demonstration of ultrathin Ge-on-insulator (GOI) inversion-type nMOSFETs, fabricated by the optimized Ge condensation technique and solid-phase diffusion of Sb. The GOI structures with low hole concentration of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">17</sup> cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> or less are realized by optimizing oxidation temperature and inserting annealing process for enhancement of intermixing in the Ge condensation process recipe. The hole concentration in the GOI body has been systematically analyzed with different oxidation temperature and insertion of annealing process. Highly doped n <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">+</sup> source/drain regions are formed in 16-nm-thick GOI layers by Sb solid-phase diffusion doping from spin-on-glass at 650°C. The high I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> /IO <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">FF</sub> ratio of 104 is observed in the fabricated nMOSFETs. The peak electron mobility of 107 cm <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /Vs is obtained for the inversion-type GOI nMOSFETs.

Experimental study of the valence band ofBi2Se3

The valence band of ${\mathrm{Bi}}_{2}{\mathrm{Se}}_{3}$ is investigated by measuring the Shubnikov--de Haas effect as well as galvanomagnetic and thermoelectric transports. At low hole concentration, the hole Fermi surface is closed and boxlike, but at higher carrier concentrations it develops tubelike extensions that are open, in general agreement with our theoretical calculations. However, the experimentally determined density-of-states effective mass is smaller than density-functional-theory calculations predict; although we cannot give a definitive explanation for this, we suspect that the theory may lack sufficient precision to compute room-temperature transport properties, such as the thermopower, in solids with interlayer van der Waals bonds.

High carrier mobility in orientation-controlled large-grain (≥50 μm) Ge directly formed on flexible plastic by nucleation-controlled gold-induced-crystallization

High-carrier-mobility semiconductors on flexible-plastic are essential to realize flexible electronics. For this purpose, electrical properties of orientation-controlled large-grain Ge crystals on flexible-plastic directly formed by nucleation-controlled gold-induced-crystallization (GIC) are examined, and compared with those obtained by aluminum-induced-crystallization (AIC). The Ge crystals show p-type conductions. Here, hole concentrations are 2.2 × 1017 and 5.8 × 1020 cm−3 for GIC-Ge and AIC-Ge, respectively, which are explained on the basis of the solubility of Au and Al in Ge. Thanks to the low hole concentration, GIC-Ge shows high hole mobility (160 cm2 V−1 s−1) compared with AIC-Ge (37 cm2 V−1 s−1). These demonstrate significant advantage of GIC to realize high-performance flexible-electronics.

Mg dopant in Cu2SnSe3: An n-type former and a promoter of electrical mobility up to 387 cm2 V−1 s−1

Mg-doped Cu2SnSe3 bulk materials with the (Cu2−xMgx)SnSe3 (Mg-x-CTSe) formula at x=0, 0.05, 0.1, 0.15, and 0.2 were prepared at 550°C for 2h with soluble sintering aids of Sb2S3 and Te. Defect chemistry was studied by measuring structural and electrical properties of Mg-doped Cu2SnSe3 as a function of dopant concentration. Mg-x-CTSe pellets show p-type at x=0, 0.05 and 0.1 and n-type at x=0.15 and 0.2. The low hole concentration of 3.2×1017cm−3 and high mobility of 387cm2V−1s−1 were obtained for (Cu2−xMgx)SnSe3 bulks at x=0.1 (5% Mg) as compared to 2.2×1018cm−3 and 91cm2V−1s−1 for the undoped one. The explanation based upon the Mg-to-Cu antisite donor defect for the changes in electrical property was declared. A high Mg content for Mg-x-CTSe at x≥0.1 can lead to the formation of second phases. The study in bulk Mg-x-CTSe has been based upon defect states and is consistent and supported by the data of structural and electrical properties.

Hexagonal boron nitride for deep ultraviolet photonic devices

This paper provides a brief overview on recent advances in tackling the doping and optical polarization issues involved in the development of high performance deep ultraviolet (DUV) light emitting devices. In particular, recent developments in the exploitation of a novel DUV emitter layer structure based on a hexagonal boron nitride (hBN) and AlGaN p–n junction and doping engineering to potentially overcome the intrinsic problem of low p-type conductivity (or low free hole concentration) in Al-rich AlGaN are summarized. By implementing the wide bandgap and highly conductive hBN p-type layer strategy in nitride DUV emitters, p-type conductivities and DUV transparency of the electron blocking layer and p-type contact layer will be dramatically increased. This will significantly improve the free hole injection and quantum efficiency, reduce the operating voltage and heat generation, and increase the device operating lifetime. The growth of undoped and Mg-doped p-type hBN via a metal organic chemical vapor deposition technique has been studied. Furthermore, p-hBN/n-AlGaN p–n junctions have been fabricated and characterized to demonstrate the feasibility and potential of p-hBN/n-AlGaN p–n heterostructure based DUV light emitting devices. Further improvements in material quality, p-type conductivity control and device processing procedures would enhance the properties of these p–n structures, which could ultimately pave the way towards the realization of high efficiency nitride DUV photonic devices.

INVESTIGATION OF GYROELECTRIC WAVEGUIDES WITH ANISOTROPIC DIELECTRIC LAYER / GIROELEKTRINIŲ BANGOLAIDŽIŲ SU ANIZOTROPINIU DIELEKTRIKO SLUOKSNIU TYRIMAS

In this paper the open, circular cross-section cylindrical gyro- electric p-GaAs (semiconductor-dielectric) waveguides with temperature-sensitive anisotropic dielectric layer are investigated. The gyroelectric phase shifters without anisotropic dielectric layer are not effective. The anisotropic layer increases the mode HE11 differential phase shift to 1060° and 1250°, when temperature T = (125–200) K, N = 5∙1019 and N = 1020 m–3 respectively. Propagated mode HE11 in gyroelectric p-GaAs waveguides, the attenuation coefficient increases, increasing the concentration of holes. The anisotropic dielectric layer reduces the mode attenuation, it is appropriate to create gyroelectric mode phase shifters with a lower hole concentration and anisotropic dielectric layer. Tiriami atvirieji, apskritojo skerspjūvio cilindriniai, giroelektriniai p-GaAs (puslaidininkiniai-dielektriniai) bangolaidžiai su jautriais temperatūrai puslaidininkine šerdimi ir anizotropiniu dielektriko sluoksniu. Šis sluoksnis padidina hibridinių pagrindinio tipo HE11 bangų diferencinį fazės pokytį iki 1060° ir 1250°, kylant temperatūrai (125–200) K ruože, kai puslaidininkio skylučių koncentracija N atitinkamai yra 5∙1019 m–3 ir 1020 m–3. Giroelektriniuose p-GaAs bangolaidžiuose HE11 bangų silpimo koeficientas didėja, didinant skylučių koncentraciją, bet anizotropinio dielektriko sluoksnis mažina bangų silpimą. Todėl, kuriant temperatūra valdomus giroelektrinius bangų fazės keitiklius, tikslinga naudoti bangolaidžius su mažesnės koncentracijos skylutėmis ir jautriu temperatūrai anizotropinio dielektriko sluoksniu.

Steady state minority carrier lifetime and defect level occupation in thin film CdTe solar cells

A model consisting of Shockley Read Hall (SRH) recombination under steady state conditions of constant photon injection is proposed in this work to study the steady state minority carrier lifetime in CdS/CdTe thin film solar cells. The SRH recombination rate versus optical injection level is analytically approximated in the junction and neutral regions. In the neutral region, it is found that the recombination rate through certain defect levels has one constant value under lower optical injection conditions and another constant value under higher optical injection conditions with the transition occurring at a critical optical injection level. By simultaneously solving the equations of charge neutrality, charge conservation and SRH recombination in the neutral region, it is found that the compensation of doping and the reduction of minority carrier lifetime by donors in the p-type semiconductor can each be remedied by optical injection. It is also demonstrated that this optical-dependent SRH recombination is significant in large bandgap thin films. The measured minority carrier diffusion length in a CdS/CdTe solar cells, as determined from the steady-state photo-generated carrier collection efficiency, shows the predicted transition of minority carrier lifetime versus optical injection level. A numerical fitting of the indirectly-measured minority carrier lifetime by assuming the minority carrier mobility gives a non-intuitive picture of the p–n junction with a low free hole concentration but a narrow depletion region width.

Colour and multicolour tuning of InGaN quantum dot based light-emitting diodes

InGaN quantum dots (QDs) formed via spinodal and binodal decomposition are used as an optically active region for light-emitting diodes (LEDs). It is shown that the emission wavelength of the electroluminescence (EL) can be shifted from blue to green by adjusting the deposition time of the InGaN QD layer. A first approach towards a monolithic multicolour emitting diode is presented. Strong difference of the EL on the InGaN QD stacking sequence is observed and is attributed to both low hole concentration and mobility.

A comprehensive picture of Cu doping in CdTe solar cells

The importance of Cu for CdTe solar cell absorber doping has been increasingly recognized in recent years. Currently different models are being discussed how to understand the case of CuCd substitutional doping in polycrystalline CdTe solar cells. In this work, an understanding is developed, which is based on a low concentration deep acceptor doped CdTe layer (Na ∼ 5 × 1014 cm−3, Ea ∼ 300 meV above the valence band). Despite their non-shallow nature, CuCd acceptors are fully or at least heavily (&amp;gt;30%) ionized. The low hole concentration in CdTe (∼1 × 1014 cm−3) originates directly from low Cu solubility in CdTe bulk material and is not caused by partial ionization or compensation as proposed by earlier models. The three to four orders of magnitude difference between bulk acceptor concentration and average Cu concentration in polycrystalline CdTe is attributed to grain boundary segregation of Cu. Our model is derived from substrate and superstrate CdTe solar cell measurements, controlled CdTe doping and quenching, Hall Effect measurements of CdTe films, numerical and analytical calculations, and a broad literature survey. Based on these results, routes to improve the conversion efficiency of CdTe solar cells are discussed.

The investigation of CuxZnSnSe4 bulks with x = 1.4–2.2 for debating the Cu excess and Cu deficiency used in thin-film solar cells

Effects of the Cu variation in CuxZnSnSe4 (CZTSe) bulks with x=1.4–2.2 on the morphological, structural, and electrical properties has been investigated. Dense CZTSe pellets with grains of 2–5μm were obtained. All CZTSe pellets showed a p-type behavior with a carrier concentration of 1016–1019cm−3. CZTSe at x=1.8 and 2.0 has a high mobility above 25cm2/Vs and an un-favored hole concentration above 1018cm−3. Although CZTSe at x=1.6 has a low hole concentration of 1.5×1017cm−3, its mobility is below 2cm2/Vs. The contradiction has made the CZTSe solar cell device with a low efficiency. The explanation, based upon vacancies and antisite defects, for the changes in electrical property is supported by the data from lattice parameter. The study in bulk CZTSe and its defects is helpful as they are applied to the fabrication of thin-film solar cell devices.