Franz–Keldysh oscillations in photoreflectance spectra of complex AlxGa1−xAs structures

Room temperature photoreflectance spectroscopy (PR) was used to investigate MBE grown Al_xGa_1−xAs/GaAs doped structures. For some structures photoreflectance spectra exhibit superposition of Franz – Keldysh oscillations. Using the 632.8 nm line from an He – Ne laser and the 457.9 nm line from an Ar⁺ laser alternately as the pump light, the Franz – Keldysh oscillations from different interfaces were separated. From the period of the oscillations, the built‐in electric field at two interfaces was determined. The dependence of the direct band‐gap energy on the Al content was also investigated. To obtain the direct band gap energy, the measured photoreflectance spectra were analysed using Aspnes lineshape procedure. The determined E^AlGaAs dependence on x for doped Al_xGa_1−xAs layers was compared with previous data. Copyright © 2000 John Wiley & Sons, Ltd.     

Systematic optical and x-ray study of In x Ga1−x As on InP

We present a systematic study of In _x Ga_1−x As on InP grown by molecular beam epitaxy using the characterization techniques of Fourier transform photoluminescence, x-ray diffraction, micro-Raman spectroscopy, and photoreflectance spectroscopy. The four techniques were used to determine and correlate the fundamental parameters of band-gap energy, phonon frequency and composition. Comparing room temperature (293 K) PL and low temperature PL indicate the presence of a partially ionized acceptor with binding energy of about 13 meV in the unintentionally doped material. Double crystal x-ray diffraction (XRD) using a symmetric <400> and asymmetric <224> reflections was also employed. The use of two reflections gives precise lattice constants, composition, and extent of film relaxation. Micro-Raman spectroscopy was used to measure phonon frequencies in the In _x Ga_1−x As films and correlated to XRD composition. Room temperature photoreflectance (PR) was used to determine band-gap energy for both the low and intermediate field cases. Band gap energies determined at room temperature by PL and PR were in agreement within experimental error.     

The properties of CuInSe2-MnSe crystals

The composition and the impurity back-ground levels of (CuInSe₂)_1-x (MnSe)_2x alloy crystals are characterized by ion microprobe analyses. Room temperature photoreflectance (PR) and photoconductivity (PC) measurements in the temperature range 10 K ⪯ T ⪯ 300 K reveal changes in the point defect chemistry and a shift of the band gap with increasingx towards higher energies. Also, a deep luminescence feature at 0.89 eV is observed in Mn doped crystals, in addition to the contributions at 0.90 and 0.94 eV that appear in both doped and nominally undoped CuInSe₂. X-ray diffraction studies show that the chalcopyrite structure is retained to at leastx = 0.25.     

Te and Ge — doping studies in Ga1−xAlxAs

The temperature dependence in the range 77–400 K of the carrier concentration, resistivity and mobility of a series of n and p-type single crystal, liquid-phase epitaxial layers of Ga_1−xAl_xAs are presented. These layers were doped, n-type with tellurium, and p-type with germanium to yield carrier concentrations in the range 10¹⁷ – 10¹⁸cm⁻³ at 295 K. Donor and acceptor ionization energies, ε_D and ε_A, are derived from the data. The dependence of ε_D on alloy composition is interpreted in terms of the known band structure variation in the alloy system.     

Zintl Phases as Thermoelectric Materials: Tuned Transport Properties of the Compounds CaxYb1–xZn2Sb2

Zintl phases are ideal candidates for efficient thermoelectric materials, because they are typically small‐bandgap semiconductors with complex structures. Furthermore, such phases allow fine adjustment of dopant concentration without disrupting electronic mobility, which is essential for optimizing thermoelectric material efficiency. The tunability of Zintl phases is demonstrated with the series Ca_xYb_1–xZn₂Sb₂ (0 ≤ x ≤ 1). Measurements of the electrical conductivity, Hall mobility, Seebeck coefficient, and thermal conductivity (in the 300–800 K temperature range) show the compounds to behave as heavily doped semiconductors, with transport properties that can be systematically regulated by varying x. Within this series, x = 0 is the most metallic (lowest electrical resistivity, lowest Seebeck coefficient, and highest carrier concentration), and x = 1 is the most semiconducting (highest electrical resistivity, highest Seebeck coefficient, and lowest carrier concentration), while the mobility is largely independent of x. In addition, the structural disorder generated by the incorporation of multiple cations lowers the overall thermal conductivity significantly at intermediate compositions, increasing the thermoelectric figure of merit, zT. Thus, both zT and the thermoelectric compatibility factor (like zT, a composite function of the transport properties) can be finely tuned to allow optimization of efficiency in a thermoelectric device.     

Reflectance and photoreflectance for in-situ monitoring of the molecular beam epitaxial growth of CdTe and Hg-based materials

Epitaxial growth of Hg-based semiconductors by molecular beam epitaxy (MBE) and metalorganic MBE (MOMBE) has progressed sufficiently to shift emphasis to the control of factors limiting the yield of both materials and devices. This paper reports on anex-situ study to evaluate the suitability of reflectance and photoreflectance (PR) asin-situ characterization techniques for the growth of CdTe and HgCdTe. Photoreflectance yields information about CdTe layers, with largest utility for doped and multi-layer structures. However, caution must be taken in interpretation of the spectra since the near-bandedge PR spectra consists of multiple transitions and the E₁ transition energy is very sensitive to the sample history. Photoreflectance appears to be of limited utility for HgCdTe single layer growth with x<0.4. However, reflectance measurements of the E₁ peak can be used to determine composition in HgCdTe single layers with an accuracy Δx = ±0.01, which can be useful for growth control. A tight binding model was used to calculate the E₁ peak energy as a function of bandgap for HgCdTe and HgTe/CdTe superlattices. Comparisons are made with experimental observations. Surface interdiffusion in HgTe-CdTe superlattices was probed using reflectance measurements.     

MOVPE growth of GaAsN: surface study by AFM and optical properties

We report on the growth of GaAs_1−xN_x thin films on GaAs substrates (2° off) by metalorganic vapor-phase epitaxy, in the temperature range 500–600°C. A mixture of N₂ and H₂ was used as the carrier gas. Using dimethylhydrazine as nitrogen source, we incorporated up to 3.5% of nitrogen, at 530°C. The growth condition dependence of nitrogen content was studied, and it reveals a distribution coefficient 350 times lower for nitrogen than for arsine at 530°C. Nitrogen incorporation is controlled by surface kinetics. The evolution of surface morphology has been investigated by atomic force microscopy as a function of the nitrogen composition and of growth temperature. For nitrogen content up to 2%, the GaAsN vicinal surface is characterized by a step–terrace structure with bunched steps, and the step edges straighten when increasing the growth temperature. For higher nitrogen content terraces are no longer observed and, above 3%, widely-spaced cross-hatch lines, characteristic of a partial relaxation of strain in the epilayers, appear. Optical properties were studied by low (7 K) and room-temperature photoluminescence and photoreflectance. As usual for this material, a degradation of optical characteristics is observed with increasing N content along with the evolution of surface morphology.     

Characterization of selective reactive ion etching effects on delta-doped GaAs/AlGaAs MODFET layers

The effects of selective reactive ion etching (SRIE) using SiCl₄/SiF₄ plasma on delta-doped GaAs/Al_0.3Ga_0.7As modulation-doped field-effect transistor (MODFET) structures and devices have been investigated. The results are compared with those of corresponding conventionally doped MODFETs. Hall measurements were conducted at 300 and 77 K to characterize the change in the transport properties of the two-dimensional electron gas due to low energy ion bombardment during the SRIE process. Delta-doped structures showed a smaller change in sheet carrier density and mobility compared to conventionally doped structures. Direct current and high frequency measurements were performed on the SRIE gate-recessed MODFETs. No significant change in threshold voltage was observed for the delta-doped MODFETs in contrast to an increase of about 300 mV for the conventionally doped MODFETs processed at a plasma self-bias voltage of −90 V and a 1200% overetch time. Maximum dc extrinsic transconductance and unity current gain cutoff frequency did not change with SRIE processing for either of the structures. This paper was presented at the Electronic Materials Conference at MIT, Cambridge, 1992.     

Performance limitation of short wavelength infrared InGaAs and HgCdTe photodiodes

The carrier lifetimes in In_xGa_1−xAs (InGaAs) and Hg_1−xCd_xTe (HgCdTe) ternary alloys for radiative and Auger recombination are calculated for temperature 300K in the short wavelength range 1.5<λ<3.7 μm. Due to photon recycling, an order of magnitude enhancements in the radiative lifetimes over those obtained from the standard van Roosbroeck and Shockley expression, has been assumed. The possible Auger recombination mechanisms (CHCC, CHLH, and CHSH processes) in direct-gap semiconductors are investigated. In both n-type ternary alloys, the carrier lifetimes are similar, and competition between radiative and CHCC processes take place. In p-type materials, the carrier lifetimes are also comparable, however the most effective channels of Auger mechanism are: CHSH process in InGaAs, and CHLH process in HgCdTe. Next, the performance of heterostructure p-on-n photovoltaic devices are considered. Theoretically predicted R_oA values are compared with experimental data reported by other authors. In_0.53Ga_0.47As photodiodes have shown the device performance within a factor often of theoretical limit. However, the performance of InGaAs photodiodes decreases rapidly at intermediate wavelengths due to mismatch-induced defects. HgCdTe photodiodes maintain high performance close to the ultimate limit over a wider range of wavelengths. In this context technology of HgCdTe is considerably advanced since the same lattice parameter of this alloy is the same over wide composition range.     

Carbon tetrachloride doped Al x Ga1−x As grown by metalorganic chemical vapor deposition

A dilute mixture of CCl₄ in H₂ has recently been shown to be a suitable carbon doping source for obtainingp-type GaAs grown by metalorganic chemical vapor deposition (MOCVD) with carbon acceptor concentrations in excess of 1 × 10¹⁹cm⁻³. To understand the effect of growth parameters on carbon incorporation in CCl₄ doped Al _x Ga_1−x As, carbon acceptor concentration was studied as a function of Al composition, growth temperature, growth rate, and CCl₄ flow rate using electrochemical capacitance-voltage profiling. The carbon incorporation as a function of Al composition, growth temperature and CCl₄ flow rate was also measured by secondary ion mass spectroscopy (SIMS). All layers were grown by low pressure MOCVD using TMGa and TMAl as column III precursors, and 100% AsH₃ as the column V source. Increased Al composition reduced the dependence of carbon concentration on the growth temperature. Reduced growth rate, which resulted in substantially decreased carbon acceptor concentrations in GaAs, had an insignificant effect on the carrier concentration of Al_0.4Ga_0.6As. A linear relationship between hole concentration and CC1₄ flow rate in Al_xGa_1−x As for 0.0 ≤x ≤ 0.8 was observed. These results are interpreted to indicate that adsorption and desorption of CCl _y (y ≤ 3) on the Al _x Ga_1-x As surface during crystal growth plays an important role in the carbon incorporation mechanism.     

Quantitative oxygen measurements in OMVPE Al x Ga1−x As grown by methyl precursors

Oxygen has always been considered to be a major contaminant in the organo-metallic vapor phase epitaxy (OMVPE) of Al _x Ga_1−x As. Oxygen incorporation has been invoked as a contributor to low luminescence efficiency, dopant compensation and degradation of surface morphology among other deleterious effects. This study presents quantitative measurements of oxygen concentration in nominally high purity Al _x Ga_1−x As. The oxygen concentration was measured as a function of alloy composition, growth temperature, andV/III ratio. Quantitative secondary ion mass spectroscopy (SIMS) measurements were used to determine the oxygen content as well as the carbon concentration in the film. The oxygen concentration increases with decreased growth temperature and V/III ratio while increasing superlinearly with Al content in the epitaxial layer.     

Influence of alloy composition,substrate temperature,and doping concentration on electrical properties of Si-doped n-Alx Ga1−x As grown by molecular beam epitaxy

Capacitance and Hall effect measurements in the temperature range 10-300 K were performed to evaluate the deep and shallow level characteristics of Si-doped n-Al_xGa_-xAs layers with 0 × 0.4 grown by molecular beam epitaxy. For alloy compositions × 0.3 the overall trap concentration was found to be less than 10⁻² of the carrier concentration. In this composition range the transport properties of the ternary alloy are comparable to those of n-GaAs:Si except for lower electron mobibities due to alloy scattering. With higher Al content one dominant electron trap determines the overall electrical properties of the material, and in n-Al_0.35Ga_0.65As:Si the deep trap concentration is already of the order of the free-carrier concentration or even higher. For the composition × = 0.35 ± 0.02 the influence of growth temperature and of Si dopant flux intensity on the deep trap concentration, on shallow and deep level activation energy, and on carrier freeze-out behaviour was studied and analyzed in detail. Our admittance measurements clearly revealed that the previously assumed deepening of the shallow level in n-Al_x Ga_1-x As of alloy composition close to the direct-indirect cross-over point does actuallynot exist. In this composition range an increase of the Si dopant flux leads to a reduction of the thermal activation energy for electron emission from shallow levels due to a lowering of the emission barrier by the electric field of the impurities. The increasing doping flux also enhances the concentration of the dominant electron trap strongly, thus indicating a participation of the dopant atoms in the formation of deep donor-type (D,X) centers. These results are in excellent agreement with the model first proposed by Lang et al. for interpretation of deep electron traps in n-Al_x Ga_1-x grown by liquid phase epitaxy.     

Temperature dependence of the optical properties of Hg1−xCdxTe

The alloy composition of Hg_1−xCd_xTe should be controlled during growth, so that the desired band gap and the lattice-matched layer may be obtained. In-situ spectroscopic ellipsometry, now commercially available, enables one to acquire spectral data during growth. If one knows the optical dielectric function as a function of alloy composition and temperature, the technique can be fully used to monitor and control temperature, the thickness, and the alloy composition. For this purpose, we first obtained temperature dependent spectral data of Hg_1−xCd_xTe by spectroscopic ellipsometry (SE). The spectral data of Hg_1−xCd_xTe with x = 1,0.235, and 0.344 were obtained from room temperature to 800Kin the photon energy range from 1.3 to 6 eV. The spectral data revealed distinctive critical point structures at E₀, E₀+Δ₀, E₁, E₁+Δ₁, E₂(X), and E₂(Σ). Critical point energies decreased and linewidths increased monotonically as temperature increased. The model for the optical dielectric function enabled (i) the critical point parameters to be determined accurately, and (ii) the spectral data to be expressed as a function of temperature within and outside the experimental range.     

Drift velocity of electrons in quantum wells of selectively doped In0.5Ga0.5As/AlxIn1 ? xAs and In0.2Ga0.8As/AlxGa1 ? xAs heterostructures in high electric fields

The field dependence of drift velocity of electrons in quantum wells of selectively doped In_0.5Ga_0.5As/Al _x In_{1 − x} As and In_0.2Ga_0.8As/Al _x Ga_{1 − x} As heterostructures is calculated by the Monte Carlo method. The influence of varying the molar fraction of Al in the composition of the Al _x Ga_{1 − x} As and Al _x In_{1 − x} As barriers of the quantum well on the mobility and drift velocity of electrons in high electric fields is studied. It is shown that the electron mobility rises as the fraction x of Al in the barrier composition is decreased. The maximum mobility in the In_0.5Ga_0.5As/In_0.8Al_0.2As quantum wells exceeds the mobility in a bulk material by a factor of 3. An increase in fraction x of Al in the barrier leads to an increase in the threshold field E _th of intervalley transfer (the Gunn effect). The threshold field is E _th = 16 kV/cm in the In_0.5Ga_0.5As/Al_0.5In_0.5As heterostructures and E _th = 10 kV/cm in the In_0.2Ga_0.8As/Al_0.3Ga_0.7As heterostructures. In the heterostructures with the lowest electron mobility, E _th = 2–3 kV/cm, which is lower than E _th = 4 kV/cm in bulk InGaAs.     

Minority carrier diffusion length in LPE InxGa1−xP: N layers (x < 0.01)

Minority carrier diffusion length In_xGa_1?xP LPE layers has been measured using the electron beam method. It has been found that this parameter significantly changes with the composition of the alloy. The measured relationship proves that minority carrier mobility in In_xGa_1?xP alloys with low indium content strongly depends on the alloy composition. It is suggested that alloy scattering may be responsible for this behaviour. The effect of doping the In_xGa_1?xP LPE layers with nitrogen has also been investigated. Results indicate that carrier mobility drops when the nitrogen concentration exceeds certain limits and this most probably can be ascribed to carrier scattering on the nitrogen centers.     

Observation of the pyroelectric effect in strained piezoelectric InGaAs/GaAs quantum-wells grown on (111) GaAs substrates

We investigated the temperature dependence of the piezoelectric constant e₁₄, i.e. the pyroelectric effect, of various strained InGaAs/GaAs single- and multi-quantum wells embedded in p-i-n structures grown on (111)B GaAs substrates and diodes made from these structures. Both photoreflectance spectroscopy and differential photocurrent spectroscopy were applied to obtain e₁₄ over the temperature range 11-300 K. The values of e₁₄ for In_xGa_1−xAs quantum well layers with x=0.12-0.21 were observed to increase with temperature, which is contrary to the expected dependence, and the strain-induced components of the pyroelectric coefficients were quantitatively determined. The dependence of the pyroelectric coefficient on In fraction is discussed.     

Variable-Field Hall Measurement and Transport in LW Single-Layer n-Type MBE Hg1−x Cd x Te

Molecular beam epitaxy n-type long-wavelength infrared (LWIR) Hg_1?x Cd _x Te (MCT) has been investigated using variable-field Hall measurement in the temperature range from 50 K to 293 K. A quantitative mobility spectrum analysis technique has been used to determine the role of multicarrier transport properties with respect to epilayer growth on lattice-matched cadmium zinc telluride, as well as lattice-mismatched silicon (Si) and gallium arsenide (GaAs) buffered substrates. Overall, after postgrowth annealing, all layers were found to possess three distinct electron species, which were postulated to originate from the bulk, transitional (or higher-x-value) regions, and an interfacial/surface layer carrier. Further, the mobility and concentration with respect to temperature were analyzed for all carriers, showing the expected mobility temperature dependence and intrinsic behavior of the bulk electron. Electrons from transitional regions were seen to match expected values based on the carrier concentration of the resolved peak. At high temperature, the lowest-mobility carrier was consistent with the properties of a surface carrier, while below 125 K it was postulated that interfacial-region electrons may influence peak values. After corrections for x-value and doping density at 77 K, bulk electron mobility in excess of 10⁵ cm² V^?1 s^?1 was observed in all epilayers, in line with expected values for lightly doped n-type LWIR material. Results indicate that fundamental conduction properties of electrons in MCT layers are unchanged by choice of substrate.     

Significantly Enhanced Thermoelectric Performance in n‐type Heterogeneous BiAgSeS Composites

High performance n‐type bulk BiAgSeS is successfully synthesized to construct heterogeneous composites which consist of mesoscale grains of both pristine BiAgSeS and doped BiAgSeS_1‐xCl_x ( x = 0.03 or 0.05). Without perceptibly deteriorating the Seebeck coefficient, a significant enhancement on electrical conductivity is obtained due to an anomalous increase of both carrier mobility and concentration; the enhanced carrier mobility is proven to be a direct result of modulation doping which relates to the band alignments, while the increased carrier concentration is attributed to the possible charge transfer from Cl rich nanoscale precipitates at the heterogeneous BiAgSeS/BiAgSeS_1‐xCl_x grain boundaries. Eventually, an enhanced figure of merit ZT ≈ 1.23 at 773 K in the composite (BiAgSeS)_0.5(BiAgSeS_0.97Cl_0.03)_0.5 is achieved, indicating that heterogeneous composites ultilizing the mechanism of modulation doping shall be a promising means of boosting the performance of thermoelectric materials. This strategy should be very likely applicable to other thermoelectrics.     

A temperature dependent model for the saturation velocity in semiconductor materials

Precise modeling of the saturation velocity is a key element for device simulation, especially for advanced devices such as e.g. High Electron Mobility Transistors (HEMTs) where the saturation velocity v_sat is directly related to the available gain of the device. We present a model implementing the temperature dependence of the saturation velocity v_sat into the two-dimensional device simulator MINIMOS-NT. The new model covers all relevant materials such as the elementary semiconductors Si and Ge, and the binary III-V group semiconductors GaAs, AlAs, InAs, GaP and InP. Furthermore, a composition dependent modeling for alloyed semiconductors such as e.g. Si_1−xGe_x, Al_xGa_1−xAs or In_xGa_1−xAs is included. The implementation reflects a comprehensive literature survey on available experimental data and Monte Carlo (MC) simulation data. The work is completed by new MC simulations, especially for material compositions, where no experimental data are available. The extraction of the saturation velocity reveals a significant difference between the saturation velocity in the bulk and the effective (saturation) velocity extracted from rf-measurements e.g. for High Electron Mobility Transistors. Since this effective value is often used for device characterization, the difference gives insight into modeling the determining quantities of HEMTs.     

OMVPE growth of p-AlGaAs/GaAs heterojunctions using diethylberyllium

We report on the OMVPE growth of modulation doped p-type Al_0.43Ga_0.57As(Be)/GaAs heterojunctions which exhibit a two-dimensional hole gas (2DHG). Hole mobilities de-termined by Hall or cyclotron resonance measurements at 300, 77, and 4 K were 394, 3750, and 21200 cm²/V bs s respectively for a sheet carrier density of about 4.5 × 10¹¹ cm^s−2. Beryllium doping of Al_xGa_1−xAs using diethylberyllium is characterized by Hall measurements, secondary ion mass spectrometry, and photoluminescence. The depen-dence of free carrier concentrationvs AlAs% forp ⁺ layers of Al_xGa_1−xAs_x,x = 0–0.5, is determined. A free carrier concentration greater than 1 × 10¹⁸ cm^s−3 is achieved forx = 0–0.43 with no carrier freeze-out down to 77 K.