Transition metal doped cadmium manganese telluride: A new material for tunable mid-infrared lasing

Relatively new materials for mid-infrared tunable lasing using chromium-doped Cd_1-xMn_xTe and cobalt-doped Cd_1-xMn_xTe have been developed. Previously, ZnS and ZnSe were used as host materials for chromium to produce mid-infrared (MIR) lasing. Compared to these materials, large diameter CdMnTe is easier to grow (using the Bridgman technique) and can be made more homogeneous. Moreover, the ternary nature of Cd_1-xMn_xTe offers the unique opportunity to optimize the optical properties of the material through variation of chemical composition and lattice parameter. Using Cd_0.55Mn_0.45Te:Cr, we have demonstrated room temperature lasing from 2.1 to 3.0 m, and we have demonstrated quasi-continuous wave (cw) lasing. To our knowledge, the observed tuning range (∼840 nm) of Cr²⁺:Cd_0.55Mn_0.45Te is the largest ever reported from a transition metal ion laser. Furthermore, this is the first time that a room temperature quasi-cw laser operating at 3 m has been demonstrated using this type of material. Also, preliminary work on Cd_0.55Mn_0.45Te:Co indicates its potential for tunable mid-infrared lasing around 3600 nm at cryogenic temperatures. Results from inductively coupled plasma mass spectrometry (ICP-MS), which determine the concentration of dopant that has been incorporated in to the host lattice, will be reported, as will the materials characterization and lasing results. The processing issues for optimizing the laser performance in these material systems will also be discussed.     

Magneto-optical Studies of Spin Phenomena in CdMnTe Doped with Co and Cr

We investigate the magneto-optical and magnetic properties of two quaternary diluted magnetic semiconductor alloys, Cd_1−x−y Mn _x Cr _y Te and Cd_1−x−y Mn _x Co _y Te, with fixed Mn concentration x ∼ 0.37 and, respectively, with concentrations of Cr in the range 0 < y < 0.07 and Co in the range␣0 < y < 0.009. The introduction of Cr and Co leads to very different behaviors, including the occurrence of ferromagnetic order in the case of Cd_1−x−y Mn _x Cr _y Te. We discuss the possible origins leading to the observed behaviors.     

Study of CdSSe:V and CdMnTe:V photorefractive effect

A large photorefractive effect was measured in CdS_0.8Se_0.2:V and Cd_0.55Mn_0.45Te:V ternary crystals which shows promise for many device applications such as optical signal processing and optical limiting. In this study, we compared the results obtained from two-wave mixing experiments with Cd_0.55Mn_0.45Te:V and CdS_0.8Se_0.2:V crystals at the 633 nm wavelength. As the signal to pump beam ratio was varied from 10⁻¹ to 10⁻⁴, Cd_0.55MN_0.45Te:V and CdS_0.8Se_0.2:V showed a maximum photorefractive gain of 0.17 and 0.20 cm⁻¹, respectively. The grating formation time of both the crystals were measured to be in the milli-second range at an incident intensity of 200 mW/cm².     

Band anticrossing in highly mismatched group II-VI semiconductor alloys

We have successfully synthesized highly mismatched Cd_1−yMn_yO_xTe_1−x alloys by high-dose implantation of O ions into Cd_1−yMn_yTe crystals. In crystals with y>0.02, incorporation of O causes a large decrease in the bandgap. The bandgap reduction increases with y; the largest value observed is 190 meV in O⁺-implanted Cd_0.38Mn_0.62Te. The results are consistent with the band anticrossing (BAC) model, which predicts that a repulsive interaction between localized states of O located above the conduction-band edge and the extended states of the conduction band causes the bandgap reduction. A best fit of the measured bandgap energies of the O-ion-synthesized Cd_1−yMn_yO_xTe_1−x alloys using the BAC model for y<0.55 suggests an activation efficiency of only ∼5% for implanted O in Cd_1−yMn_yTe.     

Infrared spectroscopy of chromium-doped cadmium selenide

The maximum optical-absorption cross section of Cr²⁺ ions was evaluated from near-infrared (NIR) absorption spectroscopy and direct measurements of the chromium concentration in Cr²⁺:CdSe crystals. The emission lifetime of the excited state, ⁵E, of Cr²⁺ was measured as a function of Cr²⁺ concentration in the 2×10¹⁷ −2×10¹⁸ ions/cm³ range and as a function of temperature from 77–300 K. Lifetime values were as high as ∼6 μs in the 77–250 K range and decreased to ∼4 μs at 300 K because of nonradiative decays. Assuming that most of the Cr dopant is in the Cr²⁺ state, an optical-absorption cross section σ_a of (1.94±0.56) × 10⁻¹⁸ cm² was calculated. Implications for laser performance are discussed.     

Absorption and photoluminescence spectroscopy of diffusion-doped ZnSe:Cr2+

ZnSe:Cr²⁺ is an attractive candidate as a room-temperature tunable solid-state laser with output in the 2–3 μm range. Passive absorption losses in this emission range currently limit laser performance. In this study, we use absorption and photoluminescence spectroscopies at 5 and 296K to address the origin of these optical losses. A series of diffusion-doped ZnSe:Cr single-crystal samples with Cr²⁺ concentrations in the range from 2×10¹⁷ cm⁻³ to 9×10¹⁹ cm⁻³ were obtained using CrSe powder as the dopant source. We find that trace amounts of Fe²⁺ produce absorption in the 2–3 μm range. Also, we have obtained data on a 680 nm absorption band observed in ZnSe:Cr which has been assigned to an internal transition of Cr²⁺. In our series of samples, the relative intensities of the 680 nm absorption band do not track the relative intensities of the 1.8 μm band (known to be due to Cr²⁺), although excitation near 680 nm does produce weak Cr²⁺ luminescence. Our absorption data do not support the current assignment of the 680 nm absorption as being an internal transition of the Cr²⁺ ion.     

四价铬离子可调谐激光器研究的进展

本文主要介绍了两种掺四价铬离子(Cr4+)可调谐激光器─-掺Cr4+镁橄榄石激光器和掺Cr4+钇铝石榴石激光器的进展,着重叙述了其光谱特性和激光特性,并对正发展中的几种新的掺Cr4+激光材料做了简要的评述。     

Molecular beam epitaxial growth of HgCdTe on CdZnTe(311)B

A series of n-type, indium-doped Hg_1−xCd_xTe (x∼0.225) layers were grown on Cd_0.96Zn_0.04Te(311)B substrates by molecular beam epitaxy (MBE). The Cd_0.96Zn_0.04Te(311)B substrates (2 cm × 3 cm) were prepared in this laboratory by the horizontal Bridgman method using double-zone-refined 6N source materials. The Hg_1−xCd_xTe(311)B epitaxial films were examined by optical microscopy, defect etching, and Hall measurements. Preliminary results indicate that the n-type Hg_1−xCd_xTe(311)B and Hg_1−xCd_xTe(211)B films (x ∼ 0.225) grown by MBE have comparable morphological, structural, and electrical quality, with the best 77 K Hall mobility being 112,000 cm²/V·sec at carrier concentration of 1.9×10⁺¹⁵ cm⁻³.     

Uniform Cr<Superscript>2+</Superscript> doping of physical vapor transport grown CdS<Subscript>x</Subscript>Se<Subscript>1−x</Subscript> crystals

The Cr²⁺ doped CdS_0.8Se_0.2 crystals were grown by the vertical, self-seeded, physical vapor transport (PVT) technique. Good quality, crack- and inclusion-free single crystals were grown with an average Cr²⁺ concentration of 5 × 10¹⁸ cm⁻³. Different source-to-tip distances were used to improve the segregation coefficient (Cr_crystal/Cr_source) of the grown crystals. It was observed that lowering the source-to-tip distance increases the segregation coefficient dramatically. With a 2-cm source-to-tip distance, good quality crystals were grown with uniform Cr²⁺ concentration throughout the ingot. The segregation coefficient was found to be ∼0.85. The composition of the crystals was also found to be fairly uniform along the length and across the diameter.     

Optical properties of Cd0.9Zn0.1 Te studied by variable angle spectroscopic ellipsometry between 0.75 and 6.24 eV

Optical properties of Cd_0.9Zn_0.1Te (CZT) were studied by variable angle spectroscopic ellipsometry (VASE). Measurements made by VASE were performed on CZT and CdTe samples in air at room temperature at multiple angles of incidence. A parametric function model was employed in the VASE analysis to determine the dielectric functions ɛ=ɛ₁+iɛ₂ in the range of 0.75 to 6.24 eV. A two-oscillator analytical model was used to describe the dielectric response of native oxides on CZT. Surface oxide optical properties and thickness on CZT were also determined in conjunction with the VASE measurement and analysis of a CdTe sample. Two samples of CZT of different oxide thicknesses were measured and their optical constants were coupled together in a multiple-sample, multiple-model VASE analysis to resolve correlations between fitting parameters. Effective medium approximation was used to describe the optical properties of the CZT oxide with roughness. A Kramers-Kronig self-consistency check of the real and imaginary parts of the Cd_0.9Zn_0.1Te dielectric functions was performed over the energy range 0.75 to 6.24 eV. A five-Lorentz-oscillator model was employed to describe the dielectric response of CZT in the range of 1.6 to 6.24 eV. Intensity transmission measurements were made on the Cd_0.9Zn_0.1Te and CdTe, showing the absorption energy band edges of ∼1.58 and 1.46 eV, respectively.     

Magnetoluminescence properties of Hg1−xCdxTe epitaxial layers and superlattice structures grown by metalorganic molecular beam epitaxy

We present a study of the electro-optical properties ofHg _1- xCd_xTe epitaxial layers and Hg_1-x Cd_xTe/CdTe (0.28 < x < 0.30) superlattice structures by x-ray diffraction, lateral transport and photo- and magneto-luminescence measurements. Systematic studies of the excitation intensity and magnetic field dependence of the photoluminescence revealed direct evidence of an excitonic contribution to the observed luminescence in Hg_{1- x}Cd_xTe epitaxial layers. Similar investigations of the superlattice structures indicated that excitonic corrections were required to adequately fit the luminescence data. Optical gains of 80 cm⁻¹ were obtained for an excitation intensity of 100 kW/cm² indicating suitable electro-optical properties for making efficient mid-infrared laser diodes.     

Far infrared characterization of Hg1-xCdxTe and related electronic materials

Far infrared (10 to 250 cm⁻¹) reflection and transmission spectroscopy is used to characterize the free-carrier and alloy properties of the leading infrared detector material Hg_1-xCd_xTe and the substrate materials CdTe and CdZnTe. The data yield values for carrier concentration and mobility, the compositional parameter x and film thickness which generally agree with other determinations. The advantages of this contactless nondestructive technique are described. Applications to the newly proposed infrared detector material Hg_1-xMn_xTe are briefly reviewed.     

Defect reduction in Hg1−xCdxTe grown by molecular beam epitaxy on Cd0.96Zn0.04Te(211)B

A study on preparation of Cd_0.96Zn_0.04Te(211)B substrates for growth of Hg_1−xCd_xTe epitaxial layers by molecular beam epitaxy (MBE) was investigated. The objective was to investigate the impact of starting substrate surface quality on surface defects such as voids and hillocks commonly observed on MBE Hg_1−xCd_xTe layers. The results of this study indicate that, when the Cd_0.96Zn_0.04Te(211)B substrates are properly prepared, surface defects on the resulting MBE Hg_1−xCd_xTe films are reduced to minimum (size, ∼0.1 m and density ∼500/cm²) so that these MBE Hg_1−xCd_x Te films have surface quality as good as that of liquid phase epitaxial (LPE) Hg_1−xCd_xTe films currently in production in this laboratory.     

Lpe growth of Hgl−xCdxTe using conventional slider boat and effects of annealing on properties of the epilayers

The epitaxial layers of Hg_1−xCd_xTe (0.17≦×≦0.3) were grown by liquid phase epitaxy on CdTe (111)A substrates using a conventional slider boat in the open tube H₂ flow system. The as-grown layers have hole concentrations in the 10¹⁷− 10¹⁸ cm⁻³ range and Hall mobilities in the 100−500 cm²/Vs range for the x=0.2 layers. The surfaces of the layers are mirror-like and EMPA data of the layers show sharp compositional transition at the interface between the epitaxial layer and the substrate. The effects of annealing in Hg over-pressure on the properties of the as-grown layers were also investigated in the temperature range of 250−400 °C. By annealing at the temperature of 400 °C, a compositional change near the interface is observed. Contrary to this, without apparent compositional change, well-behaved n-type layers are obtained by annealing in the 250−300 °C temperature range. Sequential growth of double heterostructure, Hg_l−xCd_xTe/Hg_l−yCd_yTe on a CdTe (111)A substrate was also demonstrated.     

Luminescence study of ZnTe:Cr epilayers grown by molecular-beam epitaxy

Incorporation of Cr into ZnTe epilayers grown by molecular-beam epitaxy (MBE) is reported. Photoluminescence (PL) using both continuous wave (CW) and pulsed-excitation sources is used to characterize the radiative efficiency of doped layers in the infrared region. The Cr²⁺ ions produce a broad emission band peaking in the 2–3 μm range, which is of potential use in tunable-laser devices. The optimum Cr concentration for achieving bright, room-temperature infrared emission was found to be in the range from low- to mid-10¹⁸ cm⁻³. Temperature-dependent luminescence studies were performed to determine thermal-quenching activation energies. Using a pulsed-laser operating at 1.9 μm, an investigation of emission lifetimes was made. The emission-decay curves for the Cr²⁺ recombination in ZnTe:Cr films could be described by a single exponential and were nearly independent of temperature from 80 K to 300 K. A room-temperature lifetime of ∼2.5 μsec in a ZnTe:Cr layer with [Cr] ∼1.4 × 10¹⁸ cm⁻³ compares favorably with values reported for bulk ZnTe:Cr.     

Transition-metal-doped chalcogenide lasers

Broadly tunable mid-infrared lasers are desirable for a number of scientific, remote-sensing, and military applications. Recently, Cr²⁺-doped chalcogenide lasers have emerged as an attractive source of tunable laser radiation in the 2–3.4-μm region. This paper provides a brief introduction to this field, summarizes key laser work to date, and provides examples of transmission-metaldoped lasers that operate in this region. In particular, recent results with the Cr²⁺:ZnSe laser material are highlighted.     

Automated compositional control of Hg1−xCdxTe during MBE, using in situ spectroscopic ellipsometry

The implementation of a feedback control system for maintaining a desired compositional value in Hg_1−xCd_xTe epilayers is reported. An 88-wavelength ellipsometer monitored the Cd content (x) of a Hg_1−xCd_xTe film during molecular beam epitaxy, and deviations from a pre-determined set-point were automatically corrected via adjustments in the CdTe effusion cell temperature. The accuracy of this system (Δx∼0.002) was confirmed by Fourier transform infrared transmission measurements made ex situ on the epilayers.     

Heavy Cr doping of ZnSe by molecular beam epitaxy

Epitaxial ZnSe layers were grown by molecular beam epitaxy (MBE) to study Cr incorporation with the long-term goal of demonstrating an alternate route for achieving transition-metal-doped lasers. Concentrations between 10¹⁵ atoms cm⁻³ and 4×10²⁰ atoms cm⁻³ were achieved. Secondary ion-mass spectroscopy (SIMS) concentration profiles strongly suggest that surface segregation and accumulation of Cr occurs during growth. Photoluminescence (PL) measurements indicate Cr is incorporated in the optically active Cr²⁺ state up to levels of ∼10¹⁹ cm⁻³. Electron paramagnetic resonance (EPR) studies suggest that the Cr atoms exhibit collective magnetic behavior even at these levels. X-ray diffraction (XRD) and reflection high-energy electron diffraction (RHEED) indicate high structural quality is maintained for Cr incorporation for levels up to ∼10¹⁹ atoms cm⁻³.     

Effect of the concentration of uncompensated impurities on the properties of CdTe-based X- and γ-ray detectors

Measurements of the ⁵⁵Fe-isotope emission spectra and the photosensitivity of CdTe detectors with a Schottky diode, and also the temperature dependence of the resistivity of a CdTe crystal ((2–3) × 10⁹ Ωcm at 300 K) have been used to determine the concentration of uncompensated donors (1–3) × 10¹² cm⁻³. Similar measurements performed for Cd_0.9Zn_0.1Te crystals with the resistivity (3–5) × 10¹⁰ Ω cm at 300 K have shown that the concentration of uncompensated donors in this case is lower by approximately four orders of magnitude. The results of calculations show that, due to such a significant decrease in the concentration of uncompensated donors, the efficiency of X- and γ-ray radiation detection in the photon energy range 59 to 662 keV can decrease by one-three orders of magnitude (depending on the photon energy and the lifetime of charge carriers in the space-charge region). The results obtained account for the apparent poor detecting properties of the Cd_0.9Zn_0.1Te detectors.     

The use of secondary ion mass spectrometry in the study of matrix atom diffusion in epitaxial CdTe

The self-diffusion of Cd and Te in liquid phase epitaxially grown CdTe layers has been studied using stable isotopes of Cd¹⁰⁸ and Te¹²². A two layer structure was grown from In solutions at 300–250° C, with the first layer being normal CdTe and the second enriched in Cd₁₀₈ and Te¹²². Secondary ion mass spectrometry was used to measure the concentration of the Cd and Te species as a function of depth in the as-grown structure and after heat treatment at 500° C. The main result was that after heating, no movement of Te could be detected. However, the Cd had diffused rapidly both at the solidsolid epilayer interface and at the solid-vapour interface. The self-diffusion coefficient for Cd in CdTe calculated from this data was D^500° _cd ˜ 3 ± 1 × 10¹³cm²s^-1 in reasonable agreement with published data.