Shallow impurities in quantum-well structures: A far infrared view

Recent high magnetic field, far infrared transmission and photoconductivity experiments on shallow donor impurities (Si) doped in the centers of the wells, the centers of the barriers and the edges of the wells in GaAs/AlGaAs multiple-quantum-well (MQW) structures are described. For donors doped in the centers of the wells, transitions to various high excited states have been observed. The assignment of these transitions and their relationship to the hydrogenic transitions in bulk (three dimensional) semiconductors is discussed. The edge-doped samples show dramatic differences in line shape for otherwise identical samples doped at the top and bottom of the GaAs wells. It is concluded from comparison with model calculations that the Si donors are redistributed along the growth direction, and such modeling should permit the determination of the impurity distribution non-destructively. Samples doped in the centers of the barriers show two lines at frequencies lower than that of the well-center impurities. The dominant line is due to electrons in the wells bound to their parent positive donor ions in the centers of the barriers. Possible origins of the weaker line are discussed.     

Stimulated radiation of optically pumped Cd <Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Hg<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript> Te-Based heterostructures at room temperature

The experimental observation of stimulated radiation of optically pumped Cd _x Hg_{1 ? x} Te-based heterostructures in the wavelength range of 1.4–4.5 μm is reported. In the experiments, graded-gap Cd _x Hg_{1 ? x} Te samples grown on GaAs and Si substrates by molecular beam epitaxy were used. Superluminescence of such structures was observed at 77–300 K under the pulsed pumping of the samples by a Nd:YAG laser at a wavelength of 1.064 μm. At room temperature, stimulated radiation was observed at wavelengths of 1.4–1.7 μm. The obtained experimental data are the first results on the observation of stimulated radiation from graded-gap Cd _x Hg_{1 ? x} Te structures on Si and GaAs substrates at these wavelengths at room temperature.     

Shallow donor impurities in InP bulk crystals grown by the synthesis, solute-diffusion technique

Far-infra-red photoconductivity measurements are used to study residual shallow donor species in InP bulk crystals grown by the synthesis, solute-diffusion technique. The observation of 1s-2p, m =+ 1 Zeeman transitions between the shallow donor-impurity states reveals that two dominant residual donors are included in the InP bulk crystals having carrier concentrations of less than 1 × 1015 cm?3 The donors are tentatively identified as Si and S.     

Far-infrared radiation induced photovoltage of inversion electrons on GaAs and Si

We report a novel far-infrared radiation induced photoresponse of quasi two-dimensional space-charge layers on GaAs and Si in quantizing magnetic fields. In gated AlAs---GaAs heterojunctions and in metal-oxide-silicon field-effect transistors significant changes of the gate potential are observed around even Landau level filling factors, whenever radiation of a far-infrared molecular laser is absorbed at cyclotron resonance by inversion electrons. The photosignal can be understood as reflecting the difference in the inversion electron chemical potentials at two different electron temperatures. On GaAs with far-infrared laser intensities in the regime below 1mW/cm² electron temperatures are obtained that exceed the lattice temperature by up to 10K, whereas on Si at comparable conditions changes in the electron temperature are about 2 orders of magnitude smaller.     

Effect of Si3N4 encapsulation on the laser-annealing behaviour of GaAs

When semi-insulating GaAs samples are coated with pyrolytically deposited Si3N4 layers and subsequently irradiated with a Q-switched ruby laser with energy densities > 0.3 J/cm2, it is found that a thin layer (0.1?0.25 ?m) of the underlying GaAs substrates becomes n-type. This phenomenon may be the reason why, in previous work on laser annealing of donor implanted GaAs samples, the percentage electrical activity increased when samples were coated with Si3N4.     

410～373nm激光作用下四甲基硅的多光子电离与解离

利用平行板电极装置测定了四甲基硅于某些波长点处的激光光强指数，讨论了多光子电离（ＭＰＩ）谱中某些话线的归属。通过对飞行时间（ＴＯＦ）质谱峰宽的分析并结合质谱实验结果，讨论了该分子可能的ＭＰＩ过程，得到了较高能量下Ｓｉ＋的产生仍以中性碎片的解离──硅原子的电离为主，而Ｓｉ＝（ｎ＝１～３）的形成则以中性碎片的自电离为主的结论。     

Two electron transitions of the exciton bound at the Si Donor confined in GaAs/AlxGa1-xAs quantum wells

The exciton bound to the shallow Si-donor confined in a 100A wide GaAs quantum well has been studied in selective photoluminescence (SPL) and photoluminescence excitation (PLE) spectroscopy. The transition from the ground state, ls(Γ₆), to the first excited state, 2s(Γ₆), of the confined Si donor has been observed via two-electron transitions (TETs) of the donor bound exciton observed in SPL for the first time to the best of our knowledge. The interpretation of the TET peaks is confirmed by PLE measurements. Further, from Zeeman measurements, the magnetic field dependence of the donor ls(Γ₆)-2s(Γ₆) transition energy has been determined.     

Terahertz luminescence of GaAs-based heterostructures with quantum wells under the optical excitation of donors

Spontaneous emission from selectively doped GaAs/InGaAs:Si and GaAs/InGaAsP:Si heterostructures is studied in the frequency range of ～3–3.5 THz for transitions between the states of the two-dimensional subband and donor center (Si) under the condition of excitation with a CO₂ laser at liquid-helium temperature. It is shown that the population inversion and amplification in an active layer of 100–300 cm^?1 in multilayered structures with quantum wells (50 periods) and a concentration of doping centers N _D ≈ 10¹¹ cm^?2 can be attained under the excitation-flux density 10²³ photons/(cm² s).     

超晶格光伏效应的温度特性

在18～300K 度范围内测量了 GaAs/AlGaAs 超晶格和 Ge_xSi_(1-x)/Si 应变层超晶格在不同温度下的光伏谱。在200K 以下,在 GaAs/AlGaAs 超晶格中观测到6个子带间光跃迁激子峰;在100K 以下,GaAs/AlGaAs 的光伏谱反映了超晶格台阶状态密度分布。在 Ge_xSi_1-x/Si 应变层超晶格中,观测到子带和连续带间的光跃迁。并对两类超晶格的光伏特性进行了比较分析。     

Properties of MODFET's grown on Si substrates at DC and microwave frequencies

We report the first microwave characterization of 1-µm gate GaAs/AlGaAs modulation-doped field-effect transistors (MODFET's) grown on Si substrates by MBE. Maximum transconductances of 170 mS/mm at room temperature were obtained in structures on Si, which compares to values of 200-250 mS/mm for this type of structure on GaAs. At 77 K, no collapse was observed in these structures, and transconductances of 270 mS/mm were obtained. From microwave S-parameter measurements at room temperature, current gain cutoff frequencies of 15 GHz were obtained from these GaAs/AlGaAs MOD FET's on Si, which compares with 12-15 GHz obtained on GaAs substrates. From high-frequency equivalent circuit modeling, very little difference was observed in any of the parameters between growth on Si and on GaAs. This is significant in that it demonstrates the suitability of GaAs on Si for device applications.     

Properties of structures based on laser-plasma Mn-doped GaAs and grown by MOC-hydride epitaxy

A method of doping GaAs with Mn using the laser evaporation of a metal target during MOC-hydride epitaxy is developed. The method is used to form both homogeneously doped GaAs:Mn layers and two-dimensional structures, including a δ-doped GaAs:Mn layer and a In_xGa_1?x As quantum well separated by a GaAs spacer with a thickness of d= 3–6 nm. It is shown that, at room temperature, the formed structures have magnetic and magnetooptical properties most probably caused by the presence of MnAs clusters. In the low-temperature region (～ 30 K), the anomalous Hall effect is observed. This effect is attributed to the exchange interaction between Mn ions via 2D-channel holes.     

390.60nm激光作用下Si(CH3)4的多光子解离与硅原子的(1+1)电离

在390.60nm的紫外激光作用下，利用超声分子束技术与飞行时间（TOF）质谱仪相结合的方法研究了气相四甲基硅分子多光子电离（MPI）的TOF质谱，在较低能量的激光作用下主要检测到了Si(CH3)^ 、Si^ 、C2^ 等离子的信号，有时甚至只检测到了Si^ 离子的信号：在较高能量的激光作用下主要检测到Si(CH3)^ n(n=1,2,3,4)、Si^ 、C2^ 甚至还有SiC3^ ,SiC2^ 等离子的信号。据此并结合以前得到的结论，讨论了四甲基硅分子可能的MPI过程。得出了Si^ 主要来自于Si(CH3)4的多光子解离－Si原子的（1＋1）电离、Si(CH3)n^ (n=1,2,3)的（3＋1）电离、Si(CH3)^ 4来自于Si(CH3)4的（3＋1）电离的结论。     

Frequency tuning of a CW atomic iodine laser via the Zeeman effect

A continuously operating, C₃F₇I photolytic 1.315-μm atomic iodine laser has been used to make the first precise observations of frequency tuning of an atomic iodine laser by means of the Zeeman effect. Application of a uniform magnetic field to the gain region of the photolytic iodine laser causes the laser to operate at different frequencies as a function of the strength of the applied field and the polarization of the laser. With the light polarized perpendicular to the applied magnetic field by means of Brewster output windows, the laser could be tuned to frequencies near the 3-4, 2-2, and 3-3 zero-field transitions of the hyperfine spectrum of atomic iodine. With the light polarized parallel to the applied magnetic field the laser could be tuned to two frequencies bracketing the 3-4 zero-field transition and one frequency near the 2-2 transition. Measurements show close agreement between the observed frequency behavior and theoretical models     

III–V semiconductors on Si substrates: New directions for heterojunction electronics

Excellent device performance at both d.c. and microwave frequencies has recently been obtained from GaAs based devices grown on Si substrates. In GaAs MESFETs on Si, current gain cutoff frequencies and maximum oscillation frequencies of f_T = 13.3 GHz and f_max = 30 GHz have been obtained for 1.2 μm devices, which is nearly identical to the performance achieved in GaAs on GaAs technology for both direct implant and epitaxial technology. For heterojunction bipolar transistors, current gain cutoff frequencies and maximum oscillation frequencies of f_T = 30 GHz and f_max = 11.3 GHz have been obtained for emitter dimensions of 4 × 20 μm². These results compare with the best reported HBT on GaAs substrates of f_T = 40 GHz and f_max = 26 GHz with much smaller geometry. Given the performance already demonstrated in AaAs on Si devices and the advantages afforded by this technology, the growth of III–Vs on Si promises to play an important role in the future of heterojunction electronics.     

Residual donors and acceptors in high-purity GaAs and InP grown by hydride VPE

Residual donors and acceptors in epitaxial films of GaAs and InP grown by the hydride vapor phase epitaxy technique were investigated using the complementary techniques of photothermal ionization spectroscopy and variabletemperature photoluminescence. High-purity samples of GaAs grown in three different laboratories were compared and high-purity InP samples were also measured. The dominant shallow acceptors in the GaAs samples were found to be C and Zn, and deep Cu and Mn acceptors were also observed. The donors Si, S, and Ge were observed in the GaAs with S being dominant. A clear correlation was observed between the gas phase stoichiometry during growth and the relative incorporation of column IV donors (Si and Ge) and column VI donors (S) in GaAs. Substrate quality, source purity, and atmospheric contamination of the growth system were found to influence the photoluminescence spectra of the GaAs samples. In the InP samples three shallow acceptors were observed including Zn, an unknown shallow acceptor level with E_A ≃ 21 meV, and an acceptor which may be either C or Mg. An unusual deep, structured emission band was also seen. The donors in the hydride InP were also found to be present in LPE InP and are believed to be Si and S.     

Microwave properties of self-aligned GaAs/AlGaAs heterojunction bipolar transistors on silicon substrates

(Al,Ga)As/GaAs heterojunction transistors (HBT's) grown on Si substrates have been characterized at microwave frequencies and have been found to perform extremely well. For emitter dimensions of 4 × 20 µm², current gain cutoff frequenciesf_{T} = 30GHz and maximum oscillation frequenciesf_{max}of 11.5 GHz have been obtained in a mesa-type structure. These values compare very well with thef_{T} = 40GHz andf_{max} = 26GHz which are the highest reported for HBT's on GaAs substrates in a nonmesa structure with an emitter width of ∼ 1.5 µm. These results clearly demonstrate the potential of HBT's in general at microwave frequencies, as well as the viability of GaAs on Si technology.     

Simulation of double junction In0.46Ga0.54N/Si tandem solar cell

A comprehensive study of high efficiency In_0.46Ga_0.54N/Si tandem solar cell is presented. A tunnel junction (TJ) was needed to interconnect the top and bottom sub-cells. Two TJ designs, integrated within this tandem: GaAs(n⁺)/GaAs(p⁺) and In_0.5Ga_0.5N(n⁺)/Si(p⁺) were considered. Simulations of GaAs(n⁺)/GaAs(p⁺) and In_0.5Ga_0.5N (n⁺)/Si(p⁺) TJ I-V characteristics were studied for integration into the proposed tandem solar cell. A comparison of the simulated solar cell I-V characteristics under 1 sun AM1.5 spectrum was discussed in terms of short circuit current density (J_sc), open circuit voltage (V_OC), fill factor (FF) and efficiency (η) for both tunnel junction designs. Using GaAs(n⁺)/GaAs(p⁺) tunnel junction, the obtained values of J_sc = 21.74 mA/cm², V_OC= 1.81 V, FF = 0.87 and η = 34.28%, whereas the solar cell with the In_0.5Ga_0.5N/Si tunnel junction reported values of J_sc = 21.92 mA/cm², V_OC = 1.81 V, FF = 0.88 and η = 35.01%. The results found that required thicknesses for GaAs(n⁺)/GaAs(p⁺) and In_0.5Ga_0.5N (n⁺)/Si(p⁺) tunnel junctions are around 20 nm, the total thickness of the top InGaN can be very small due to its high optical absorption coefficient and the use of a relatively thick bottom cell is necessary to increase the conversion efficiency.     

Domain epitaxial growth of TiN/Si(001), TiN/GaAs(001), and Si/TiN/Si(001) heterostructures by laser physical vapor deposition: Theory and experiment

We have successfully deposited epitaxial titanium nitride films on (001) silicon and (001) gallium arsenide substrates and multilayer Si/TiN/Si(001) epitaxial heterostructures using pulsed laser (KrF: λ = 248 nm, τ = 25 ns) physical vapor deposition. The deposition of TiN was carried out at a substrate temperature of 600°C on Si(001) and 400°C on GaAs(00l). The interfaces were sharp without any indication of interfacial reaction. The epitaxial relationships were found to be <001> TiN ‖<001> Si on the silicon substrate, <001> Si ‖<001> TiN |<001> Si on the heterostructure, and [1-10] TiN‖[110] GaAs and [001] TiN ‖[110] GaAs on the GaAs substrate. The growth in these large-mismatch systems is modeled and the various energy terms contributing to the growth of these films are determined. The domain matching epitaxy provides a mechanism of epitaxial growth in systems with large lattice mismatch.The epitaxial growth is characterized by domain epitaxial orientation relationships with m lattice constants of epilayer matching with n of the substrate and with a small residual domain mismatch present in the epilayer. This residual mismatch is responsible for a coherent strain energy. The magnitude of compression of Ti-N bond in the first atomic layer, contributing to the chemical free energy of the interface during the initial stages of growth, is found to be a very important factor in determining the orientation relationship. This result was used to explain the differences in the orientaion relationships between TiN/Si and TiN/GaAs systems. The various energy terms associated with the domain epitaxial growth are evaluated to illustrate that the domain epitaxial growth is energetically favorable compared to the lattice mismatched epitaxial growth. The results of this analysis illustrate that the observed variations in the epitaxial growth are consistent with the minimum energy configurations associated with the domain epitaxial growth.     

New infrared laser lines in argon, krypton, and xenon

Fourteen new infrared laser transitions in Ar, Kr, and Xe ranging from3.725 to 17.233 mum are reported. All of the Kr laser transitions and all but one of the Ar and Xe transitions have upper levels in thesordstate: the majority of the transitions satisfy the conditionDelta K = Delta J. Identification of a previously observed, unidentified laser line at 5.804 μm in Ar is given.     

Generation and the characteristics of current pulses from alloyed-type low-conductivity bulk GaAs

Current pulses with frequencies in the vicinity of 1 kHz were observed in low-conductivity copper-doped GaAs. The coherent current pulses were obtained by applying an ac half-sinusoid voltage to low-conductivity bulk GaAs samples with indium ohmic contacts. The repetition rate increased as the applied voltage was increased. Illumination of the specimen surface or change in ambient temperature had a strong effect on the current pulses. The current pulses are due to ionization of deep levels and are compared to the microplasma pulses observed in Si and Ge p-n junctions.