CdTe Substrates Research Articles

UV photon assisted control of interface charge between CdTe substrates and metalorganic chemical vapor deposition CdTe epilayers

The technology to control the interface charge density between CdTe substrates and CdTe epilayers grown by metalorganic chemical vapor deposition is studied. The interface charge is determined by the modified built-in potential derived from capacitance-voltage characteristics of Schottky contacts formed on the epilayers. Novel ultraviolet (UV) photon assisted and photo thermal surface pretreatments that control the interface between p-type CdTe substrates and CdTe epilayers are reported. The substrates are exposed to UV radiation provided by a high pressure Hg lamp operating at 600 W with a wide emission spectrum between 190 and 300 nm. The UV photon assisted surface pretreatment with hydrogen is compared with additional surface pretreatments: thermal pretreatment with hydrogen (without UV photons) and UV photo thermal pretreatment with hydrogen. The UV photon assisted and the UV photo thermal surface pretreatments with hydrogen reduce the interface charge density to a practically negligible value. In addition, the p-type doping level of the substrate is reduced considerably in a layer of few microns adjacent to the interface.

Synchrotron white beam x-ray topography analysis of MBE grown CdTe/CdTe (111)B

The structural quality of CdTe(111)B substrates and MBE grown CdTe epilayers is examined with synchrotron white beam x-ray topography (SWBXT). Reflection SWBXT indicates that CdTe substrates with comparable x-ray double crystal rocking curve full width at half maximum values can have radically different defect microstructures, i.e. dislocation densities and the presence of inclusions. Dislocation mosaic structures delineated by SWBXT are consistent with the distribution of etch pits revealed by destructive chemical etch pit analysis. Direct one-to-one correspondence between distinct features of the topographic image and individual etch pits is demonstrated. Clearly resolved images of individual dislocations are obtained by carrying out transmission SWBXT. Our investigation demonstrates how, the extent of twinning in a CdTe epilayer is strongly influenced by the quality of the defect microstructure, and how dislocations propagate from an inclusion.

Hall effect characterization of LPE HgCdTe P/n heterojunctions

The field and temperature dependence of the Hall coefficient has been used to simultaneously extract information about the p and n layers in very long wave length infrared P/n HgCdTe heterojunctions. The field dependence allows the effects of high mobility electrons to be separated from those of low mobility holes. The higher the magnetic field, the higher the sensitivity to the parameters of the P layer. For a maximum magnetic field of 8000 gauss, the hole sheet concentration must be at least five times the electron sheet concentration to obtain accurate results for the P layer. This criterion is satisfied for typical liquid phase epitaxy (LPE) heterostructures. The analysis determines the hole sheet resistance (concentration times mobility), rather than the hole concentration or mobility separately. Independent knowledge of the P layer thickness and the relationship between hole concentration and resistivity are needed to convert the Hall measurement results to hole concentrations. Analysis of the field-dependent Hall data is complicated by the finding that at least three electrons of different mobilities are needed to fit the field dependence of the Hall coefficient in n-type LPE HgCdTe layers. These results are consistent with previous conclusions that electrons with different mobilities are needed to model bulk n-HgCdTe, and with a range of mobilities in the graded composition interface between the LPE layer and CdTe substrate. Consistent results are obtained for the concentrations and mobilities of the three types of electrons in the n-HgCdTe layer with and without the P layer present. N and P type carrier concentrations are also consistent with dopant concentrations measured by secondary ion mass spectroscopy.

Substrate issues for the growth of mercury cadmium telluride

Close lattice matching and lattice compatibility with mercury cadmium telluride (MCT) make CdTe and related alloys ideal substrate materials for growth of MCT layers for the purpose of making high-performance second-generation infrared detectors. However, the limitations in the properties of CdTe and the difficulties in its bulk growth have prompted extensive research in the area of alternative substrates. Some basic relevant characteristics of substrates such as sapphire, GaAs, and silicon are compared and the possibilities and problems associated with each material are analyzed in the light of the most recent results in the field.

Growth of MgTe and Cd 1− xMg xTe thin films by molecular beam epitaxy

We report on the growth of the compound semiconductor MgTe as well as the ternary alloy Cd 1− x Mg x Te by molecular beam epitaxy. This is to our knowledge the first time that this material has been grown by any epitaxial technique. Bulk MgTe, which is hygroscopic, has a band gap of 3.0 eV and crystallizes usually in the wurtzite structure. Pseudomorphic films were grown on zincblende CdTe substrates for a MgTe thickness below a critical layer thickness of approximately 500 nm. In addition, Cd 1- x Mg x Te epilayers were grown with a Mg concentration between 0 and 68%, which corresponds to a band gap between 1.5 and 2.5 eV at room temperature. The crystalline quality of the layers is comparable to CdTe thin films as long as they are fully strained. The lattice constant of zincblende MgTe is slightly smaller than that of CdTe, and the lattice mismatch is as low as 0.7%. In addition highly n-type CdMgTe layers were fabricated by bromine doping. The tunability of the band gap as well as the rather good lattice match with CdTe makes the material interesting for optoelectronic device applications for the entire visible range.

Optically pumped laser action in double-heterostructure HgCdTe grown by metalorganic chemical vapor deposition on a CdTe substrate

Photopumped laser action in a HgCdTe double-heterostructure grown by metalorganic chemical vapor deposition on a CdTe substrate containing (311)- and (211)-oriented grains was studied. The (311)-oriented device exhibited laser action around λ=4 μm with a threshold power increasing exponentially from 56 mW at T=12 K to 2.8 W at the highest lasing temperature of 90 K. The (211)-oriented laser device emitted around 2.5 μm. The threshold power of the (211) device was much higher than that of the (311) one (1.5 W at 12 K) and increased with temperature at a lower rate to 4.8 W at the highest lasing temperature of 110 K. Front illumination photoluminescence from both active and cladding layers of the (311)-oriented heterostructure was studied as a function of temperature.

Oxygen on the (100) CdTe surface

We have investigated oxygen on CdTe substrates by means of x-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED). A Te oxide layer that was at least 15 Å thick was found on the surface of as-delivered CdTe substrates that were mechanically polished. This oxide is not easily evaporated at temperatures lower than 350 °C. Furthermore, heating in air, which further oxidizes the CdTe layer, should be avoided. Etching with HCl acid (15% HCl) for at least 20 s and then rinsing with de-ionized water reduces the Te oxide layer on the surface down to 4% of a monoatomic layer. However, according to XPS measurements of the O 1s peak, 20%–30% of a monoatomic layer of oxygen remains on the surface, which can be eliminated by heating at temperatures ranging between 300 and 340 °C. The RHEED patterns for a molecular beam epitaxially (MBE)-grown CdTe film on a (100) CdTe substrate with approximately one monoatomic layer of oxidized Te on the surface lose the characteristics of the normal RHEED patterns for a MBE-grown CdTe film on an oxygen-free CdTe substrate.

Non-destructive characterization of HgCdTe using photoinduced microwave reflection

The authors present the details of a photoinduced microwave reflectance set-up operating at 35 GHz for measuring the excess carrier lifetime and surface recombination velocity of semiconductor epitaxial layers or bulk wafers. The principles upon which the operation of the set-up is based are given in detail, pointing out those situations in which the measurements can give erroneous results. Evaluation of HgCdTe epitaxial layers grown in the laboratory by MOCVD on semi-insulating CdTe substrates have shown excess carrier lifetimes of 1.1 mu s and a surface recombination velocity of 1400 cm s-1 at 77 K.

Optically pumped laser action and photoluminescence in HgCdTe layer grown on (211) CdTe by metalorganic chemical vapor deposition

Photopumped laser action and photoluminescence (PL) in HgCdTe layers grown by metalorganic chemical vapor deposition on (211) oriented CdTe substrates were studied as a function of temperature in the wavelength range 2.5–3.3 μm. Lasing was observed up to 160 K. External quantum efficiency of 5% and single-mirror peak power of 14 mW were measured at 12 K for a 500-μm-long device. The external quantum efficiency was measured for different laser cavity lengths and the results were used to derive the internal efficiency and the laser loss. HgCdTe layers covered with a CdTe cap layer exhibited a lower PL intensity and laser devices with higher threshold.

Photoluminescence of n-type CdTe:I grown by molecular beam epitaxy

Liquid helium temperature and temperature-dependent photoluminescence (PL) spectroscopy have been performed on highly n-type-doped CdTe:I films grown by molecular beam epitaxy. The samples were grown on 2° off (001) oriented bulk CdTe substrates at substrate temperatures from 170 to 250 °C, and exhibit room-temperature electron concentrations of 1×1017 cm−3. The brightest PL edge emission at liquid helium and room temperatures was observed from a sample grown at 210 °C. At T=5 K, the iodine donor radiative recombination was observed at 1.593 eV, corresponding to a donor ionization energy of 14 meV for the ITe substitutional donor, in agreement with the predicted hydrogenic donor ionization energy for CdTe. The thermal quenching behavior of the edge emission peak is a two-step process involving both 10 and 14 meV activation energies. These activation energies are related to the thermalization of the ITe donor from the ground state to the first excited state (1s→2s, 10 meV), and complete thermalization from the donor ground state to the conduction band (14 meV).

Growth and characterization of CdTe, HgTe and HgCdTe by atomic layer epitaxy

We present the first demonstration of atomic layer epitaxy (ALE) of HgTe, CdTe and HgCdTe using Hg, Cd and Te alkyl chemistry on GaAs and CdTe substrates. The ALE deposition experiments were done at atmospheric pressure in a horizontal reactor equipped with a fast switching manifold. Methylallytelluride, dimethylmercury (DMHg) and dimethylcadmium were used for Te, Hg and Cd sources respectively. ALE of HgTe was achieved at 140°C, and the monolayer per cycle condition extended over a wide range of DMHg flux. ALE growth of CdTe was carried out over a wide temperature range (250–290°C) and reactant partial pressure. HgCdTe layers were also grown by alternately depositing HgTe and CdTe onto CdTe substrates and then interdiffusing them at higher temperatures.

Photon assisted growth of HgTe by metalorganic chemical vapor deposition

Photon assisted metalorganic chemical vapor deposition (MOCVD) of HgTe by the reaction of mercury and diethyltelluride (DETe) on CdTe substrates is reported. The growths were done at 380–450°C and subatmospheric pressure, with helium as the carrier gas. A simple kinetic model for the MOCVD growth is presented. The model yields the growth rate as a function of the gas-phase partial pressures of the constituents in various limiting cases, and is corroborated by experimental results. The apparent activation energy for growth without photons (33–49 kcal/mol) exceeds the activation energy of pyrolysis of DETe (∼25 kcal/mol) and is significantly reduced in photon assisted growth (18–28 kcal/mol). The effect of photon assisted growth on morphology, thickness uniformity, and location along the susceptor is also presented and discussed.

Unusual strain caused by the twin boundary structure in CdTe(1̄1̄1̄) films

Strain due to the twin boundary structure in CdTe(1̄1̄1̄) films was analyzed. CdTe(1̄1̄1̄) films were grown on GaAs(100) and CdTe(1̄1̄1̄) substrates. High density twinned domains existed in the films. The strain and lattice constant of the films were measured by optical reflectance and X-ray diffraction analyses, respectively. Crystalline lattices of the films examined were dominantly under biaxial compressive stress. It is found that the amount of strain is in proportion to the defect density. The strain is only slightly relaxed for the films with twinned domains caused by double positioning. The strain is enhanced by defects characteristic of CdTe(1̄1̄1̄) films.

Photon assisted reduction of interface charge between CdTe substrates and metalorganic chemical vapor deposition CdTe epilayers

A novel photothermal in situ surface pretreatment that reduces the interface charge between p-type CdTe substrates and CdTe epilayers grown by metalorganic chemical vapor deposition is reported. Prior to the actual growth of the epilayers, the substrates are held at 450 °C in hydrogen atmosphere and simultaneously exposed to ultraviolet radiation. The interface charge is determined by the modified builtin potential derived from capacitance–voltage characteristics of Schottky contacts formed on the epilayers. The interface charge density is reduced from a typical value of ∼1×1012 cm−2 to a practically negligible value.

Laser growth of CdTe epitaxial film on CdTe substrate

The growth of CdTe epitaxial film on a CdTe substrate using a laser deposition method has been investigated: the beam of a pulsed ArF excimer laser is focused on a CdTe target and the plume of ejected material is collected onto a (111) Zndoped CdTe substrate (4%). From the X-ray diffraction measurements and double-diffraction X-ray analysis, the mirror-like film grown is found to be a homoepitaxial layer. Film thicknesses, deduced from optical profilometry, range between 1.9 and 6.5μm. The few differences that appeared between the photoluminescence spectra of the film and the substrate can be explained by a small composition variation and some defects or impurities in the CdTe that lead to new radiative transitions.

Observation of magnetophonon resonance oscillations in open-gap Hg1-x-yCdxMnyTe grown by LPE

Open-gap n-type Hg1-x-yCdxMnyTe crystals have been successfully grown on CdTe substrates by the LPE method from Hg-rich solutions. Using the high-quality crystals with y=0.01-0.02 grown by this method, the authors have succeeded in observing the magnetophonon resonance oscillations and the magnetic quantum oscillations due to the Auger recombination process. These oscillations are analysed based on the modified Pidgeon-Brown model. In both types of the oscillations, the temperature dependence of the observed peak positions is found to be stronger than those predicted by the model which includes the temperature dependence of the energy gap Eg. The origin of their anomaly is discussed.

Piezomodulated reflectivity on CdMnTe/CdTe quantum well structures as a new standard characterization method

Piezomodulated reflectivity (PZR) measurements are reported for the first time as a standard characterization method for CdMnTe/CdTe single (SQW) and multiple (MQW) quantum wells grown by molecular beam epitaxy on CdTe substrates 1 mm thick. Previously, modulation spectroscopy studies of II–VI structures required thin substrates which needed special preparation. In this paper we present studies of optical properties of CdMnTe/CdTe SQWs and MWQs using the PZR technique. The samples, mounted on a sinusoidally driven piezoelectric transducer are subjected to an alternating strain. Exploiting “lock-in” techniques, the first derivative of the reflectivity is measured directly. Specific electronic transitions, e.g. excitons, are well resolved in the modulated spectrum and can be easily identified. This makes PZR a very sensitive and powerful tool for the characterization of quantum well structures, and a useful complement to other standard techniques such as photoluminescence and excitation spectroscopy.

Nanosecond and Femtosecond UV Laser Ablation of CdTe (100): Time-Of-Flight and Angular Distributions

AbstractAngularly resolved time-of-flight (TOF) measurements have been used to probe the velocity and angular distributions of Cd atoms and Te2 molecules ejected from CdTe (100) substrates under irradiation by 248 nm nanosecond and sub-picosecond laser pulses. These experiments employ a dye laser TOF mass spectrometer with resonance enhanced multiphoton ionization for sensitive, high resolution detection of the desorbed products. The velocity distributions are well described by Maxwell-Boltzmann distributions for low fluence nanosecond (<60 mJ/cm2) and sub-picosecond (<3.3 mJ/cm2) pulses. Angular flux distributions for nanosecond irradiation are observed to be highly forward peaked about the surface normal, whereas, for sub-picosecond irradiation the distribution approaches cos3θ.

Assessment of doped CdxHg1-xTe structures using bevelled sections

Doped homo- and heterostructures of CdxHg1-xTe (CMT) are required for some types of new-generation devices. These structures have been grown by metal organic vapour phase epitaxy (MOVPE) at approximately 360 degrees C onto both CdTe and GaAs (buffered with CdTe) substrates. The structures were produced using both acceptor and donor doping with arsenic and iodine respectively, and the junctions formed following annealing in mercury to eliminate metal vacancies. This paper summarizes progress in characterizing these junctions both in terms of x and electrical behaviour.

Heteroepitaxy of CdTe on GaAs and silicon substrates

CdTe can be grown directly by molecular beam epitaxy on substrates such as GaAs or silicon, which exhibit very large lattice mismatches of 14.6% and 19% respectively. The occurrence of dual epitaxy, which has been previously reported for growth on (100)GaAs, has also been found recently for growth on (211)GaAs. The (133)CdTe-(211)GaAs hetero-interface presents a smooth continuation of the tetrahedral bond network from GaAs to CdTe, which is not the case for the (211)CdTe-(211)GaAs interface. Single-domain, twin-free CdTe(111)B films are currently obtained on Si(100) surface where single atomic steps are dominant. The crystalline quality of CdTe/Si films has been dramatically improved as confirmed by X-ray diffraction, photoluminescence and electron microscopy investigations. The narrowest rocking curves obtained for as-grown epilayers are 70 arcsec for (133)CdTe/(211)GaAs, 50 arcsec for a flash-annealed (211)CdTe/(211)GaAs and 140 arcsec for (111)B CdTe/(100)Si. These results confirm that the CdTe/GaAs and CdTE/Si composite substrates should be viewed as prime candidates to replace bulk CdTe substrates.