EPR studies of point defects in Cu-III–VI2 chalcopyrite semiconductors

EPR and PL spectra were measured to investigate point defects in I–III–VI₂ type chalcopyrite semiconductors where the group I element is Cu. Taking into account various optical spectra, the EPR signals observed were assigned to defect centers involving residual Fe impurities and Cu-vacancies. Some of the point defects were found to form defect-complexes.     

Temperature dependence of band gaps in HgCdTe and other semiconductors

Band-edge shifts induced by the electron-phonon interaction are calculated for HgCdTe alloys and various semiconductor compounds starting from accurate zero-temperature band structures. The calculated temperature variation of gaps agrees with experiments to better than 10% in all materials except InAs and InSb where the deviation is about 50%. While the simple picture that the intra (inter)-band transitions reduce (increase) the gap still holds, we show that both the conduction band edge E_c and valence band edge E_v move down in energy. These shifts in E_v affect the valence band offsets in heterojunctions at finite temperature. The temperature variations of valence band offset and the electron effective mass are also reported.     

Ultrahigh Stability 3D TI Bi2Se3/MoO3 Thin Film Heterojunction Infrared Photodetector at Optical Communication Waveband

Infrared (IR) detection at 1300–1650 nm (optical communication waveband) is of great significance due to its wide range of applications in commerce and military. Three dimensional (3D) topological insulator (TI) Bi₂Se₃ is considered a promising candidate toward high‐performance IR applications. Nevertheless, the IR devices based on Bi₂Se₃ thin films are rarely reported. Here, a 3D TI Bi₂Se₃/MoO₃ thin film heterojunction photodetector is shown that possesses ultrahigh responsivity (R_i), external quantum efficiency (EQE), and detectivity (D*) in the broadband spectrum (405–1550 nm). The highest on–off ratio of the optimized device can reach up to 5.32 × 10⁴. R_i, D*, and the EQE can reach 1.6 × 10⁴ A W^?1, 5.79 × 10¹¹ cm² Hz^1/2 W^?1, and 4.9 × 10⁴% (@ 405 nm), respectively. Surprisingly, the R_i can achieve 2.61 × 10³ A W^?1 at an optical communication wavelength (@ 1310 nm) with a fast response time (63 µs), which is two orders of magnitude faster than that of other TIs‐based devices. In addition, the device demonstrates brilliant long‐term (>100 days) environmental stability under environmental conditions without any protective measures. Excellent device photoelectric properties illustrate that the 3D TI/inorganic heterojunction is an appropriate way for manufacturing high‐performance photodetectors in the optical communication, military, and imaging fields.     

T(z) diagram and optical energy gap values of Cd1−zMnzGa2Se4 alloys

The T(z) diagram of the system Cd_1−zMn_zGa₂Se₄ was obtained from x-ray diffraction and differential thermal analysis measurements. It was found that at lower temperatures, a single phase solid solution occurs across the whole compositional range and values of lattice parameters were determined as a function of z. At higher temperatures, an order-disorder transition occurs, in the range 0 < z < 0.6 to a partially ordered tetragonal structure and for 0.6 < z < 1. 0 to a disordered defect zinc-blende structure. In the T(z) diagram, both the ordering boundary and the solidus curve appear to show discontinuities at z = 0.6, corresponding to the change in the disordered phase. It is suggested that the symmetries of the terminal compounds are different one from the other. Optical absorption measurements were made at 300 K to show the variation of the direct optical energy gap E_o with z, and again the values appear to divide into two parts below the above z = 0.6.     

Transport Properties of Se/As2Se3 Nanolayer Superlattice Fabricated Using Rotational Evaporation

Since their proposal by Esaki, superlattices have been observed to have fascinating features such as quantum size effects, negative differential resistance, and sequential resonant tunneling. However, the technology threshold for fabricating superlattices is high, requiring methods like molecular beam epitaxy (MBE) and atomic layer deposition (ALD), among others, even for amorphous materials. Thus, the desirable features from superlattices have not been extensively utilized. It is shown that superlattices of Se and As₂Se₃ (superlattice‐Se), fabricated using rotational evaporation, exhibit sequential tunneling, a typical superlattice feature. From current–voltage measurements of the superlattice deposited on n‐type Si, oscillations in the characteristics are observed. Using models of reverse‐biased Schottky barriers, the observations are explained as tunneling in sequence from superlattice minibands. The superlattice‐Se also shows carrier blocking, with a resistivity of the order of 10¹² Ω cm in dark conditions at room temperature, despite the low resistivity (≈10 Ω cm) of the n‐type Si substrate. When the Si is illuminated, the device shows higher detectivity for weaker signals compared to higher illumination. The ease of fabrication, and the blocking and amplifying capabilities make superlattice‐Se an interesting “add‐on” structure to improve conventional photodetector materials such as Si or Ge, which have issues with dark currents at room temperature.     

WO3薄膜的后氧处理及其红外反射率研究

本文采用“改进电沉积法”制备ＷＯ３薄膜,并研究了该薄膜的后氧处理方法,以及后氧处理条件对薄膜的电化学特性,尤其是红外反射率的影响。结果表明,经该方法处理过的薄膜,其红外反射率调制能力提高８％。     

T(z) phase diagram and optical energy gaps in CuGa(Se z Te1−z )2 alloys

TheT(z) phase diagram of the system CuGa(Se _z Te_1-z )₂ was obtained from x-ray diffraction and differencial thermal analysis (DTA) measurements. DTA measurements were carried out on each sample and the transition temperatures were plotted as a function of alloy composition. Values of lattice parameter were determined in all cases. Values of the optical energy gap at 300 K were obtained from optical absorption measurements, and these were analysed by using a recently proposed model which explains the downward bowing observed in the I-III-VI₂ alloys.     

Comparison of dry etching of III-V semiconductors in ICl/Ar and IBr/Ar electron cyclotron resonance plasmas

ICl/Ar and IBr/Ar plasmas were found to be promising candidates for room temperature dry etching processing of the III-V semiconductors GaAs, AlGaAs, GaSb, InP, InGaAs, and InSb. Results showed fast etch rates (0.7 μm/min in ICl/Ar and −0.3 μm/min in IBr/Ar), and at high microwave power, 1000 W, good surface morphology (typical root mean square roughness ∼2 nm), while retaining the near-surface stoichiometry, especially in IBr/Ar plasmas. There was little change of surface smoothness over a wide range of plasma compositions for Gacontaining materials in both ICl/Ar and IBr/Ar plasmas, (e.g. GaAs), while there was a window region with about 25–50% of IBr in IBr/Ar plasmas to maintain good morphology of In-containing semiconductors like InP. Selectivities of 4–10 over mask materials such as SiO₂, SiN_x, and W were typical in ICl/Ar plasmas.     

使用Na_2CO_3/NaHCO_3体系进行单晶硅制绒

通过单晶硅表面制绒可以降低太阳能电池面板对光的反射率进而提高电池的转化效率。利用Na2CO3、NaHCO3和Na2CO3/NaHCO3溶液进行了单晶硅表面制绒,并利用扫描电子显微镜和紫外/可见光光度计对所得绒面的形貌及反色率分别进行了分析。研究了溶液浓度、溶液组成及反应时间等因素对所制单晶硅绒面反射率的影响。结果表明:在Na2CO3:NaHCO3质量比为10:1,反应温度为85℃,反应时间为10 min时,可获得反射率为15.22%的绒面。     

Etching of As- and P-based III–V semiconductors in a planar inductively coupled BCl<Subscript>3</Subscript>/Ar plasma

A parametric study of the etch characteristics of Ga-based (GaAs, GaSb, and AlGaAs) and In-based (InGaP, InP, InAs, and InGaAsP) compound semiconductors in BCl₃/Ar planar inductively coupled plasmas (ICPs) was performed. The Ga-based materials etched at significantly higher rates, as expected from the higher volatilities of the As, Ga, and Al trichloride, etch products relative to InCl₃. The ratio of BCl₃ to Ar proved critical in determining the anisotropy of the etching for GaAs and AlGaAs, through its effect on sidewall passivation. The etched features in In-based materials tended to have sloped sidewalls and much rougher surfaces than for GaAs and AlGaAs. The etched surfaces of both AlGaAs and GaAs have comparable root-mean-square (RMS) roughness and similar stoichiometry to their unetched control samples, while the surfaces of In-based materials are degraded by the etching. The practical effect of the Ar addition is found to be the ability to operate the ICP source over a broader range of pressures and to still maintain acceptable etch rates.     

(Bi2Te3)0.90(Sb2Te3)0.05(Sb2Se3)0.05热压合金微观结构和电学性能相关性研究

通过熔炼,研磨制备N型(Bi2Te3)0．90(Sb2Te3)0．05(Sb2Se3)0．05热电材料的粉末,热压制备混合粉末热压合金。通过SEM和XRD研究热压合金的微观结构,在室温测量热压合金样品的电学性能。结果表明热压合金在微观结构和电学性能上存在各向异性,从而预示能够在增强材料机械强度的同时提高其热电性能。     

Observation of lasing from Cr2+:CdTe and compositional effects in Cr2+-doped II-VI semiconductors

We are engaged in a systematic study of the optical and laser properties of Cr²⁺-doped cadmium chalcogenides. Previously, we demonstrated quasi-continuous wave lasing from Cr²⁺-doped Cd_0.55Mn_0.45Te with slope efficiencies as high as 64% and a laser tuning range from 2,170–3,010 nm. In this paper, we report the first demonstration of lasing from Cr:CdTe at room temperature. Pulsed-laser operation was obtained with a free-running spectrum centered at 2,535 nm. The slope efficiency of the laser was low (∼1%) because of large parasitic losses at the laser wavelength. The spectroscopic properties of Cr:CdTe are favorable for laser applications because of a large emission cross section (∼2.5 × 10⁻¹⁸ cm²) and a high emission-quantum yield (∼88%). In addition, CdTe can easily incorporate Cr ions either through melt growth or diffusion doping. Along with our results on Cr²⁺:CdTe, we report on the optical properties of several other Cr²⁺-doped II-VI semiconductors (ZnS, ZnSe, ZnTe, CdS, CdSe, CdTe, Cd_0.9Zn_0.1Te, Cd_0.65Mg_0.35Te, Cd_0.85Mn_0.15Te, and Cd_0.55Mn_0.45Te) and compare them for applications as solid-state laser materials.     

Formation and characterization of CdS/ZnSiAs2 heterojunctions

A rectifying junction between MOCVD formed ZnSiAs₂ and evaporated CdS has been studied. Current-voltage behavior at current densities above about 1mA/cm² appears to be dominated by tunnelling. Capacitance measurements indicate an acceptor concentration in ZnSiAs₂ of 5 × 10¹⁶cm^-3, junction diffusion voltage of 1.0V and the presence of deep traps. Device series resistance could be attributed in part to compensating cross-doping. CdS/ZnSiAs₂ junctions exhibit photovoltaic response, and photovoltages above 0.5V under 5 sun illumination. ZnSiAs₂ electron affinity is estimated to be 3.9eV, and a tentative CdS/ZnSiAs₂ band diagram is presented.     

Elastic and optoelectronic properties of novel Ag3AuSe2 and Ag3AuTe2 semiconductors

Ag₃AuSe₂ and Ag₃AuTe₂ are interesting class of semiconducting materials. Here, elastic and opto-electronic properties of Ag₃AuSe₂ and Ag₃AuTe₂ semiconductors are studied in detail using density functional theory. Different schemes are selected to treat the exchange-correlation effects. The unit cell of the compounds is fully optimized and calculated cell constants are found in agreement to the existing experimental data. The calculated elastic constants and elastic moduli reveal that the compounds possess ductile nature and are elastically stable. It is found that both compounds are direct bandgap semiconductor with bandgap value of 1.009 eV for Ag₃AuSe₂ and 0.551 eV for Ag₃AuTe₂. As the compounds have narrow and direct bandgaps, therefore optically active. The optical properties like reflectivity, absorption coefficient, energy loss function, refractive index including and complex dielectric function are studied in detail. The direct band gap nature of these compounds make them useful candidate for different devices applications.     

Mechanical and electronic properties of Si,Ge and their alloys in P42/mnm structure

Structural, mechanical, and electronic properties of Si–Ge alloys in P4₂/mnm structure were studied using first-principles calculations by Cambridge Serial Total Energy Package (CASTEP) plane-wave code. The calculations were performed with the local density approximation and generalized gradient approximation in the form of Perdew–Burke–Ernzerhof, PBEsol. The calculated excess mixing enthalpy is positive over the entire germanium composition range. The calculated formation enthalpy shows that the Si–Ge alloys are unstable at 0 K; however, the alloys might exist at specified high temperature scale. The anisotropic calculations show that Si₁₂ in P4₂/mnm structure exhibits the greatest anisotropy in Poisson’s ratio, shear modulus, Young’s modulus and the universal elastic anisotropy index A^U, but Si₈Ge₄ has the smallest anisotropy. The electronic structure calculations reveal that Si₁₂ and Si–Ge alloys in P4₂/mnm structure are indirect band gap semiconductors, but Ge₁₂ in P4₂/mnm structure is a direct semiconductor.     

EPR studies of native and impurity-related defects in II–IV–V2 semiconductors

Magnetic resonance studies of native defects in Zinc germanium diphosphide (ZnGeP₂) and their energy level positions in the band gap are reviewed and first results on defects in ZnSiP₂ are presented. The contribution of the dominant native defects to the optical absorption of ZnGeP₂ crystals is revealed by photo-EPR investigations in combination with the first ODMR experiments. The EPR results published for Mn and Fe in the II–IV–V₂ chalcopyrites are summarized. The observation of some new Mn centers and the first experimental detection of antiferromagnetic Mn–Mn coupled pairs in ZnGeP₂ are presented. In addition, new results concerning Mn-induced local changes of the free parameter x_f of the chalcopyrite structure at the impurity site are discussed.     

Al2O3 stacks on In0.53Ga0.47As substrates: In situ investigation of the interface

In this work we present an in situ investigation of the interface composition between an In_0.53Ga_0.47As substrate and an Al₂O₃ oxide grown by molecular beam deposition in ultra high vacuum conditions. In the effort to improve the chemical quality of the interface, reduction of semiconductor-oxygen bonding at the interface can be obtained by growing a few Å thick pure Al layer before starting exposure of the surface to the atomic oxygen flux. Conversely, when a Ge interface passivation layer is intercalated between the semiconductor and the oxide stack, the interface chemistry is governed by Ge reaction with other species (Al, O), leading only to a partial suppression of the interface oxides.     

Surface mobility and distribution of electrons in the accumulation layer of Ga2Se3/GaAs heterostructures

The dependence of the electron drift mobility in the undepleted conduction channels of Ga₂Se₃/GaAs heterostructures on the surface charge density is measured. The presence of charge coupling in the accumulation layer sufficient for creating electrical (or microelectronic) devices is discovered. Fiz. Tekh. Poluprovodn. 32, 718–720 (June 1998)     

Investigation of pyroelectric characteristics of 0.8 Pb (Zn1/3Nb2/3)O3 - 0.1 Pb TiO3 - 0.1 BaTiO3 ceramics with special reference to uncooled infrared detection

The pyroelectric characteristics for 0.8 Pb(Zn_1/3Nb_2/3)O₃ - 0.1 PbTiO₃ - 0.1 BaTiO₃ ceramics were investigated to determine the usefulness of this material as an uncooled infrared detector. Two successive phase transitions, changing the crystal symmetry from rhombohedral to tetragonal, and then to cubic on a heating process, were observed in the material. The results of measurements were compared with those of PLZT (8/40/60) and the Ca-Modified PbTiO₃, which are currently the favored materials for uncooled arrays. The results indicate that 0.8 PZN - 0.1 PT - 0.1 BT is a promising material for both transverse mode and pyroelectric CCD arrays.